const ILLEGAL_EXP : &'static str = "Illegal Expression";
const ERR_PARSING_NOT_MATCH : &'static str = "Error parsing expression! \
Must be a number or operator (+,-,* or /)";
const ERR_POSTFIX_INCOMPLETE : &'static str = "Postfix expression incomplete!";
const ILLEGAL_EXP : &'static str = "Illegal Expression";
const ERR_PARSING_NOT_MATCH : &'static str = "Error parsing expression! \
Must be a number or operator (+,-,* or /)";
const ERR_POSTFIX_INCOMPLETE : &'static str = "Postfix expression incomplete!";