don't regenerate script value

The checking for variables used subsequently recalls the function used to get the fresh variable value on the inherited environment. This is undesirable if the script code has side effects. This change memoizes the result of the script code. It is not clear that this is the correct approach if the variable is bound in many places, but it seems to be what is done in the basic fresh id case that prompts the user. Overall the result in the script case seems to be no better or worse than the result in the prompt case.