This function parses the input string using the Lisp parser, and returns the first complete expression found in the string. The rest of the string is ignored.
Gamma> a = parse_string("hello");
hello
Gamma> b = parse_string("(cos 5)");
(cos 5)
Gamma> c = parse_string("(+ 5 6) (/ 6 3)");
(+ 5 6)
Gamma> eval(b);
0.28366218546322624627
Gamma> eval(c);
11
Gamma>