This an implementation of a Top-Down parser that supports variables, functions,
basic operators and constants. Is deep tested and seems to works fine. Enjoy.

sl_parser_evaluate (string)	evaluate a string and prints the result
sl_parser_evaluate_to_double 	returns the results but don't prints anything

this parser supports:
	constants (builtin)
	user variables
	functions (builtin)
	many kind of errors	
