added "not"

input.scm

@@ -52,3 +52,4 @@ x
 
 (define a1 (make-account 100.00))
 (a1 -20.00)
+

proc.go

@@ -83,3 +83,10 @@ func division(vals []interface{}) (interface{}, error) {
 	}
 	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
+}

skeam.go

@@ -27,6 +27,7 @@ var universe = &environment{map[symbol]interface{}{
 	"-":      builtin(subtraction),
 	"*":      builtin(multiplication),
 	"/":      builtin(division),
+	"not":    builtin(not),
 	"define": special(define),
 	"quote":  special(quote),
 	"if":     special(_if),

special.go

@@ -71,6 +71,13 @@ func quote(_ *environment, args ...interface{}) (interface{}, error) {
 	return args[0], nil
 }
 
+func booleanize(v interface{}) bool {
+	if b, ok := v.(bool); ok {
+		return b
+	}
+	return true
+}
+
 // defines the built-in "if" contruct.  e.g.:
 //
 //  (if #t "foo" "bar")
@@ -90,11 +97,11 @@ func _if(env *environment, args ...interface{}) (interface{}, error) {
 		return nil, err
 	}
 
-	if b, ok := v.(bool); ok && !b {
+	if booleanize(v) {
-		return eval(args[2], env)
-	}
 		return eval(args[1], env)
 	}
+	return eval(args[2], env)
+}
 
 // defines the built-in "set" construct, which is used to set the value of an
 // existing symbol in the provided environment.  e.g.: