added "symbol?"

input.scm

@@ -70,3 +70,6 @@ x
 
 (null? (quote ()))
 (null? (list))
+
+(symbol? (quote null))
+(symbol? 1)

proc.go

@@ -132,3 +132,12 @@ func isnull(vals []interface{}) (interface{}, error) {
 
 	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
+}

skeam.go

@@ -21,9 +21,12 @@ func (s sexp) String() string {
 type symbol string
 
 var universe = &environment{map[symbol]interface{}{
+	// predefined values
 	"#t":   true,
 	"#f":   false,
 	"null": nil,
+
+	// builtin functions
 	"+":       builtin(addition),
 	"-":       builtin(subtraction),
 	"*":       builtin(multiplication),
@@ -33,6 +36,9 @@ var universe = &environment{map[symbol]interface{}{
 	"list?":   builtin(islist),
 	"not":     builtin(not),
 	"null?":   builtin(isnull),
+	"symbol?": builtin(issymbol),
+
+	// special forms
 	"begin":  special(begin),
 	"define": special(define),
 	"if":     special(_if),