Form calls the user edit routine with the type parameter equal
to U_BEGINVAL every time the operator presses the VALIDATE key and form
enters a validation form. The old_value parameter points to the name
of the current form, and the new_value parameter points to the name of
the validation form. The exit_char parameter contains the form number
of the validation form. The edit_name parameter is NULL. If you return
an error message, the validation form is not displayed and the error message
is shown.