This is a standard number evaluator which does not allow the trig functions sin, cos, tan, sec, csc, cot in the answer, and does not allow a decimal answer. Useful if you want to require an answer of the form 1/sqrt(2) or pi/2.
ans_string: A string which evaluates to a number
Produces an answer evaluator which checks student's answer as a number. Student's answer cannot contain one of the trig functions or a decimal point
an answer evaluator