added "names"

i'm just making shit up now.

builtin.go

@@ -228,3 +228,11 @@ var cdr = builtin{
 		return s.items[1:], nil
 	},
 }
+
+var names = builtin{
+	name:  "names",
+	arity: 0,
+	fn: func(vals []interface{}) (interface{}, error) {
+		return universe.keys(), nil
+	},
+}

env.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"fmt"
+	"sort"
 )
 
 type UnknownSymbolError struct{ symbol }
@@ -40,6 +41,15 @@ func (e environment) set(key symbol, val interface{}) {
 	e.items[key] = val
 }
 
+func (e environment) keys() []string {
+	keys := make([]string, 0, len(e.items))
+	for key, _ := range e.items {
+		keys = append(keys, string(key))
+	}
+	sort.Strings(keys)
+	return keys
+}
+
 func (e environment) defined(key symbol) bool {
 	_, err := e.get(key)
 	return err == nil

skeam.go

@@ -85,6 +85,10 @@ var universe = &environment{map[symbol]interface{}{
 	symbol(set.name):      set,
 }, nil}
 
+func init() {
+	universe.set(symbol(names.name), names)
+}
+
 // parses the string lexeme into a value that can be eval'd
 func atom(t token) (interface{}, error) {
 	switch t.t {