from Stacks import StackArray

def infix_to_postfix(infixexpr):
    """Converts an infix expression to an equivalent postfix expression """

    """Signature:  a string containing an infix expression where tokens are space separated is
       the single input parameter and returns a string containing a postfix expression
       where tokens are space separated"""
    
    ????? = StackArray(30)
...
    tokenList = infixexpr.split()
...
    return " ".join(postfixList)


def postfix_eval(postfixExpr):
    """  Purpose """

def doMath(op, op1, op2):
    """  Purpose """

def postfix_valid(postfixexpr):
    """ Purpose """
