diff --git a/acc.go b/acc.go
index 4e35b58..80b3966 100644
--- a/acc.go
+++ b/acc.go
@@ -5,6 +5,9 @@ import (
 	"reflect"
 )
 
+// type accumulator describes an accumulator.  That is, it is a numerical
+// structure that applies a pair of functions across a list of values that are
+// expected to be numerical; i.e. of type int64 or float64.
 type accumulator struct {
 	name     string
 	floatFn  func(float64, float64) (float64, error)
@@ -14,7 +17,7 @@ type accumulator struct {
 	floating bool
 }
 
-func (a *accumulator) total(vals []interface{}) (interface{}, error) {
+func (a accumulator) total(vals []interface{}) (interface{}, error) {
 	if vals == nil || len(vals) == 0 {
 		return a.acc, nil
 	}
diff --git a/builtin.go b/builtin.go
new file mode 100644
index 0000000..193e471
--- /dev/null
+++ b/builtin.go
@@ -0,0 +1,235 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+type builtin struct {
+	// name of the function
+	name string
+
+	// minimum number of arguments
+	arity int
+
+	// whether we will accept arbitrarily large numbers of arguments or not
+	variadic bool
+
+	// function to be called
+	fn func([]interface{}) (interface{}, error)
+}
+
+// begins by evaluating all of its inputs.  An error on input evaluation will
+// stop evaluation of a builtin.  After evaluating its inputs, an arity check
+// is performed to see if the proper number of arguments have been supplied.
+// Perhaps this is the wrong order, I'm unsure.  Finally, the procudure is
+// passed the post-evaluation arguments to be executed.
+func (b builtin) call(env *environment, rawArgs []interface{}) (interface{}, error) {
+	// eval all arguments first
+	args := make([]interface{}, 0, len(rawArgs))
+	for _, raw := range rawArgs {
+		v, err := eval(raw, env)
+		if err != nil {
+			return nil, err
+		}
+		args = append(args, v)
+	}
+
+	if err := b.checkArity(len(rawArgs)); err != nil {
+		return nil, err
+	}
+
+	return b.fn(args)
+}
+
+func (b builtin) checkArity(n int) error {
+	if n == b.arity {
+		return nil
+	}
+
+	if b.variadic && n > b.arity {
+		return nil
+	}
+
+	return arityError{
+		expected: b.arity,
+		received: n,
+		name:     b.name,
+		variadic: b.variadic,
+	}
+}
+
+var add = builtin{
+	name:     "+",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return accumulator{
+			name: "+",
+			floatFn: func(left, right float64) (float64, error) {
+				return left + right, nil
+			},
+			intFn: func(left, right int64) (int64, error) {
+				return left + right, nil
+			},
+		}.total(vals)
+	},
+}
+
+var sub = builtin{
+	name:     "-",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return accumulator{
+			name: "-",
+			floatFn: func(left, right float64) (float64, error) {
+				return left - right, nil
+			},
+			intFn: func(left, right int64) (int64, error) {
+				return left - right, nil
+			},
+		}.total(vals)
+	},
+}
+
+var mul = builtin{
+	name:     "*",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return accumulator{
+			name: "*",
+			floatFn: func(left, right float64) (float64, error) {
+				return left * right, nil
+			},
+			intFn: func(left, right int64) (int64, error) {
+				return left * right, nil
+			},
+			acc:  1,
+			accf: 1.0,
+		}.total(vals)
+	},
+}
+
+var div = builtin{
+	name:     "/",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return accumulator{
+			name: "/",
+			floatFn: func(left, right float64) (float64, error) {
+				if right == 0.0 {
+					return 0.0, errors.New("float division by zero")
+				}
+				return left / right, nil
+			},
+			intFn: func(left, right int64) (int64, error) {
+				if right == 0 {
+					return 0, errors.New("int division by zero")
+				}
+				return left / right, nil
+			},
+		}.total(vals)
+	},
+}
+
+var not = builtin{
+	name:  "not",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return !booleanize(vals[0]), nil
+	},
+}
+
+var length = builtin{
+	name:  "length",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		switch t := vals[0].(type) {
+		case sexp:
+			return len(t), nil
+		case list:
+			return len(t.sexp), nil
+		}
+		return nil, fmt.Errorf("first argument must be sexp, received %v", reflect.TypeOf(vals[0]))
+	},
+}
+
+var lst = builtin{
+	name:     "list",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return list{sexp(vals), 1}, nil
+	},
+}
+
+var islist = builtin{
+	name:  "list?",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		switch vals[0].(type) {
+		case list, sexp:
+			return true, nil
+		}
+		return false, nil
+	},
+}
+
+var isnull = builtin{
+	name:  "null?",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		s, ok := vals[0].(sexp)
+		if !ok {
+			return false, nil
+		}
+		return len(s) == 0, nil
+	},
+}
+
+var issymbol = builtin{
+	name:  "symbol?",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		_, ok := vals[0].(symbol)
+		return ok, nil
+	},
+}
+
+var cons = builtin{
+	name:  "cons",
+	arity: 2,
+	fn: func(vals []interface{}) (interface{}, error) {
+		s := sexp{vals[0]}
+		switch t := vals[1].(type) {
+		case sexp:
+			return append(s, t...), nil
+		default:
+			return append(s, t), nil
+		}
+		panic("not reached")
+	},
+}
+
+var car = builtin{
+	name:  "car",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		s, ok := vals[0].(sexp)
+		if !ok {
+			return nil, errors.New("expected list")
+		}
+		return s[0], nil
+	},
+}
+
+var cdr = builtin{
+	name:  "cdr",
+	arity: 1,
+	fn: func(vals []interface{}) (interface{}, error) {
+		s, ok := vals[0].(sexp)
+		if !ok {
+			return nil, errors.New("expected list")
+		}
+		return s[1:], nil
+	},
+}
diff --git a/cmp.go b/cmp.go
new file mode 100644
index 0000000..76e4147
--- /dev/null
+++ b/cmp.go
@@ -0,0 +1,101 @@
+package main
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+)
+
+type cmp_bin_i func(int64, int64) bool
+type cmp_bin_f func(float64, float64) bool
+
+func cmp_left(vals []interface{}, fni cmp_bin_i, fnf cmp_bin_f) (bool, error) {
+	if len(vals) < 2 {
+		return false, errors.New("expected at least 2 arguments")
+	}
+
+	var lasti int64
+	var lastf float64
+	var floating bool
+
+	switch v := vals[0].(type) {
+	case float64:
+		floating = true
+		lastf = v
+	case int64:
+		lasti = v
+	default:
+		return false, fmt.Errorf("gt is not defined for %v", reflect.TypeOf(v))
+	}
+
+	for _, raw := range vals[1:] {
+		switch v := raw.(type) {
+		case float64:
+			if !floating {
+				floating = true
+				lastf = float64(lasti)
+			}
+			if !fnf(lastf, v) {
+				return false, nil
+			}
+			lastf = v
+		case int64:
+			if floating {
+				f := float64(v)
+				if !fnf(lastf, f) {
+					return false, nil
+				}
+				lastf = f
+			} else {
+				if !fni(lasti, v) {
+					return false, nil
+				}
+				lasti = v
+			}
+		default:
+			return false, errors.New("ooga booga")
+		}
+	}
+
+	return true, nil
+}
+
+var gt = builtin{
+	name:     ">",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		fni := func(x, y int64) bool { return x > y }
+		fnf := func(x, y float64) bool { return x > y }
+		return cmp_left(vals, fni, fnf)
+	},
+}
+
+var gte = builtin{
+	name:     ">=",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		fni := func(x, y int64) bool { return x >= y }
+		fnf := func(x, y float64) bool { return x >= y }
+		return cmp_left(vals, fni, fnf)
+	},
+}
+
+var lt = builtin{
+	name:     "<",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		fni := func(x, y int64) bool { return x < y }
+		fnf := func(x, y float64) bool { return x < y }
+		return cmp_left(vals, fni, fnf)
+	},
+}
+
+var lte = builtin{
+	name:     "<=",
+	variadic: true,
+	fn: func(vals []interface{}) (interface{}, error) {
+		fni := func(x, y int64) bool { return x <= y }
+		fnf := func(x, y float64) bool { return x <= y }
+		return cmp_left(vals, fni, fnf)
+	},
+}
diff --git a/proc.go b/proc.go
deleted file mode 100644
index bd25e8e..0000000
--- a/proc.go
+++ /dev/null
@@ -1,257 +0,0 @@
-package main
-
-import (
-	"errors"
-	"fmt"
-	"reflect"
-)
-
-type builtin func([]interface{}) (interface{}, error)
-
-// evaluates all of the arguments, and then calls the function with the results
-// of the evaluations
-func (b *builtin) call(env *environment, rawArgs []interface{}) (interface{}, error) {
-	if rawArgs == nil {
-		return (*b)(nil)
-	}
-
-	// eval all arguments first
-	args := make([]interface{}, 0, len(rawArgs))
-	for _, raw := range rawArgs {
-		v, err := eval(raw, env)
-		if err != nil {
-			return nil, err
-		}
-		args = append(args, v)
-	}
-
-	return (*b)(args)
-}
-
-func addition(vals []interface{}) (interface{}, error) {
-	a := accumulator{
-		name: "addition",
-		floatFn: func(left, right float64) (float64, error) {
-			return left + right, nil
-		},
-		intFn: func(left, right int64) (int64, error) {
-			return left + right, nil
-		},
-	}
-	return a.total(vals)
-}
-
-func subtraction(vals []interface{}) (interface{}, error) {
-	a := accumulator{
-		name: "subtraction",
-		floatFn: func(left, right float64) (float64, error) {
-			return left - right, nil
-		},
-		intFn: func(left, right int64) (int64, error) {
-			return left - right, nil
-		},
-	}
-	return a.total(vals)
-}
-
-func multiplication(vals []interface{}) (interface{}, error) {
-	a := accumulator{
-		name: "multiplication",
-		floatFn: func(left, right float64) (float64, error) {
-			return left * right, nil
-		},
-		intFn: func(left, right int64) (int64, error) {
-			return left * right, nil
-		},
-		acc:  1,
-		accf: 1.0,
-	}
-	return a.total(vals)
-}
-
-func division(vals []interface{}) (interface{}, error) {
-	a := accumulator{
-		name: "division",
-		floatFn: func(left, right float64) (float64, error) {
-			if right == 0.0 {
-				return 0.0, errors.New("float division by zero")
-			}
-			return left / right, nil
-		},
-		intFn: func(left, right int64) (int64, error) {
-			if right == 0 {
-				return 0, errors.New("int division by zero")
-			}
-			return left / right, nil
-		},
-	}
-	return a.total(vals)
-}
-
-func not(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "not"); err != nil {
-		return nil, err
-	}
-	return !booleanize(vals[0]), nil
-}
-
-func length(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "length"); err != nil {
-		return nil, err
-	}
-
-	x, ok := vals[0].(sexp)
-	if !ok {
-		return nil, fmt.Errorf("first argument must be sexp, received %v", reflect.TypeOf(vals[0]))
-	}
-	return len(x), nil
-}
-
-func lst(vals []interface{}) (interface{}, error) {
-	return sexp(vals), nil
-}
-
-func islist(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "list?"); err != nil {
-		return nil, err
-	}
-
-	_, ok := vals[0].(sexp)
-	return ok, nil
-}
-
-func isnull(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "null?"); err != nil {
-		return nil, err
-	}
-
-	s, ok := vals[0].(sexp)
-	if !ok {
-		return false, nil
-	}
-
-	return len(s) == 0, nil
-}
-
-func issymbol(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "symbol?"); err != nil {
-		return nil, err
-	}
-
-	_, ok := vals[0].(symbol)
-	return ok, nil
-}
-
-func cons(vals []interface{}) (interface{}, error) {
-	if err := checkArity(2, vals, "cons"); err != nil {
-		return nil, err
-	}
-	s := sexp{vals[0]}
-	switch t := vals[1].(type) {
-	case sexp:
-		return append(s, t...), nil
-	default:
-		return append(s, t), nil
-	}
-	panic("not reached")
-}
-
-func car(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "car"); err != nil {
-		return nil, err
-	}
-	s, ok := vals[0].(sexp)
-	if !ok {
-		return nil, errors.New("expected list")
-	}
-	return s[0], nil
-}
-
-func cdr(vals []interface{}) (interface{}, error) {
-	if err := checkArity(1, vals, "cdr"); err != nil {
-		return nil, err
-	}
-	s, ok := vals[0].(sexp)
-	if !ok {
-		return nil, errors.New("expected list")
-	}
-	return s[1:], nil
-}
-
-type cmp_bin_i func(int64, int64) bool
-type cmp_bin_f func(float64, float64) bool
-
-func cmp_left(vals []interface{}, fni cmp_bin_i, fnf cmp_bin_f) (bool, error) {
-	if len(vals) < 2 {
-		return false, errors.New("expected at least 2 arguments")
-	}
-
-	var lasti int64
-	var lastf float64
-	var floating bool
-
-	switch v := vals[0].(type) {
-	case float64:
-		floating = true
-		lastf = v
-	case int64:
-		lasti = v
-	default:
-		return false, fmt.Errorf("gt is not defined for %v", reflect.TypeOf(v))
-	}
-
-	for _, raw := range vals[1:] {
-		switch v := raw.(type) {
-		case float64:
-			if !floating {
-				floating = true
-				lastf = float64(lasti)
-			}
-			if !fnf(lastf, v) {
-				return false, nil
-			}
-			lastf = v
-		case int64:
-			if floating {
-				f := float64(v)
-				if !fnf(lastf, f) {
-					return false, nil
-				}
-				lastf = f
-			} else {
-				if !fni(lasti, v) {
-					return false, nil
-				}
-				lasti = v
-			}
-		default:
-			return false, errors.New("ooga booga")
-		}
-	}
-
-	return true, nil
-}
-
-func gt(vals []interface{}) (interface{}, error) {
-	fni := func(x, y int64) bool { return x > y }
-	fnf := func(x, y float64) bool { return x > y }
-	return cmp_left(vals, fni, fnf)
-}
-
-func gte(vals []interface{}) (interface{}, error) {
-	fni := func(x, y int64) bool { return x >= y }
-	fnf := func(x, y float64) bool { return x >= y }
-	return cmp_left(vals, fni, fnf)
-}
-
-func lt(vals []interface{}) (interface{}, error) {
-	fni := func(x, y int64) bool { return x < y }
-	fnf := func(x, y float64) bool { return x < y }
-	return cmp_left(vals, fni, fnf)
-}
-
-func lte(vals []interface{}) (interface{}, error) {
-	fni := func(x, y int64) bool { return x <= y }
-	fnf := func(x, y float64) bool { return x <= y }
-	return cmp_left(vals, fni, fnf)
-}
diff --git a/skeam.go b/skeam.go
index 3126713..0f30711 100644
--- a/skeam.go
+++ b/skeam.go
@@ -41,25 +41,25 @@ var universe = &environment{map[symbol]interface{}{
 	"null": nil,
 
 	// builtin functions
-	"+":       builtin(addition),
-	"-":       builtin(subtraction),
-	"*":       builtin(multiplication),
-	"/":       builtin(division),
-	">":       builtin(gt),
-	">=":      builtin(gte),
-	"<":       builtin(lt),
-	"<=":      builtin(lte),
-	"cons":    builtin(cons),
-	"car":     builtin(car),
-	"cdr":     builtin(cdr),
-	"length":  builtin(length),
-	"list":    builtin(lst),
-	"list?":   builtin(islist),
-	"not":     builtin(not),
-	"null?":   builtin(isnull),
-	"symbol?": builtin(issymbol),
-	// "="
-	// "equal?"
+	symbol(add.name):      add,
+	symbol(sub.name):      sub,
+	symbol(mul.name):      mul,
+	symbol(div.name):      div,
+	symbol(gt.name):       gt,
+	symbol(gte.name):      gte,
+	symbol(lt.name):       lt,
+	symbol(lte.name):      lte,
+	symbol(cons.name):     cons,
+	symbol(car.name):      car,
+	symbol(cdr.name):      cdr,
+	symbol(length.name):   length,
+	symbol(lst.name):      lst,
+	symbol(islist.name):   islist,
+	symbol(not.name):      not,
+	symbol(isnull.name):   isnull,
+	symbol(issymbol.name): issymbol,
+	// "=":       builtin(equal),
+	// "equal?":  builtin(equal),
 	// "eq?"
 	// "append"
 
diff --git a/special.go b/special.go
index eac1898..6936128 100644
--- a/special.go
+++ b/special.go
@@ -17,9 +17,15 @@ type arityError struct {
 	expected int
 	received int
 	name     string
+	variadic bool
 }
 
 func (n arityError) Error() string {
+	if n.variadic {
+		return fmt.Sprintf(`received %d arguments in *%v*, expected %d (or more)`,
+			n.received, n.name, n.expected)
+	}
+
 	return fmt.Sprintf(`received %d arguments in *%v*, expected %d`,
 		n.received, n.name, n.expected)
 }
@@ -31,10 +37,10 @@ func checkArity(arity int, args []interface{}, name string) error {
 		if arity == 0 {
 			return nil
 		}
-		return arityError{arity, 0, name}
+		return arityError{arity, 0, name, false}
 	}
 	if len(args) != arity {
-		return arityError{arity, len(args), name}
+		return arityError{arity, len(args), name, false}
 	}
 	return nil
 }