arc> ($.pair? '(1))
  #t

  > (%.pair? '(1))
  #t

You'll need to add one line to ac.scm, and then:

  arc> (ac-scheme.pair? '(1))
  #t

If you start using this you'll start finding the need for some of the other functions in that link, which transform data back and forth to the way racket or arc likes it. Come back and ask us more questions when you run into errors like this:

  arc> (ac-scheme.length '(1 2 3))
  Error: "length: contract violation\n  expected: list?\n  given: '(1 2 3 . nil)"

  cc -o mtl-arc -std=c11 mtl-arc.c
  mtl-arc.c: In function ‘new_sym’:
  mtl-arc.c:46:2: warning: implicit declaration of function ‘strdup’ [-Wimplicit-function-declaration]
    symname(result) = strdup(sym);
    ^
  mtl-arc.c:46:18: warning: assignment makes pointer from integer without a cast [enabled by default]
    symname(result) = strdup(sym);
                    ^
  mtl-arc.c: In function ‘new_string’:
  mtl-arc.c:59:20: warning: assignment makes pointer from integer without a cast [enabled by default]
    stringval(result) = strdup(string);
                      ^
  mtl-arc.c: In function ‘new_builtin’:
  mtl-arc.c:118:15: warning: assignment makes pointer from integer without a cast [enabled by default]
    help(result) = strdup(doc);
                 ^
  mtl-arc.c: In function ‘char_to_token’:
  mtl-arc.c:170:2: warning: return makes pointer from integer without a cast [enabled by default]
    return strdup(cbuf);
    ^
  mtl-arc.c: In function ‘split_string’:
  mtl-arc.c:194:20: warning: assignment makes pointer from integer without a cast [enabled by default]
      *(result + i++) = strdup(token);
                      ^
  mtl-arc.c: In function ‘buf_to_string’:
  mtl-arc.c:212:2: warning: return makes pointer from integer without a cast [enabled by default]
    return strdup(buf);
    ^
  mtl-arc.c: In function ‘read_expr’:
  mtl-arc.c:327:4: warning: passing argument 1 of ‘split_string’ makes pointer from integer without a cast [enabled by default]
      char **nums = split_string(strdup(token), '/');
      ^
  mtl-arc.c:181:8: note: expected ‘char *’ but argument is of type ‘int’
   char **split_string(char *a_str, const char a_delim) {
          ^
  /tmp/ccMru0iN.o: In function `builtin_cos':
  mtl-arc.c:(.text+0x3b0e): undefined reference to `sin'
  /tmp/ccMru0iN.o: In function `builtin_expt':
  mtl-arc.c:(.text+0x3bd2): undefined reference to `pow'
  /tmp/ccMru0iN.o: In function `builtin_log':
  mtl-arc.c:(.text+0x3c53): undefined reference to `log'
  /tmp/ccMru0iN.o: In function `builtin_rand':
  mtl-arc.c:(.text+0x3db2): undefined reference to `floor'
  /tmp/ccMru0iN.o: In function `builtin_sin':
  mtl-arc.c:(.text+0x3e46): undefined reference to `sin'
  /tmp/ccMru0iN.o: In function `builtin_sqrt':
  mtl-arc.c:(.text+0x3ec7): undefined reference to `sqrt'
  /tmp/ccMru0iN.o: In function `builtin_tan':
  mtl-arc.c:(.text+0x3f48): undefined reference to `tan'
  /tmp/ccMru0iN.o: In function `builtin_trunc':
  mtl-arc.c:(.text+0x3fc9): undefined reference to `trunc'
  collect2: error: ld returned 1 exit status
  make: *** [all] Error 1

Among other things, I recently added a prototype curly-infix reader, which has been kind of fun coding with, because it makes math bearable, and it makes certain things easier to read. Along with variable negation, it seems quite handy.

examples:

(def isa (a b) {type.a is b}) => (def isa (a b) (is (type a) b))

{caar = [car car._]} => (= caar (fn (_) (car (car _))))