Form calls the user edit routine with the type parameter equal
to U_ENTERFIELD just before the operator begins typing data into a field
that has the user edit field set to yes. The edit_name parameter
points to the name specified in the edit field name field in the field description
form. If the edit field name is blank, then the edit_name parameter points
to the field name. The old_value parameter points to the original contents
of the field when the operator first entered the field.