The INPUT method of VPGL Language get a user input.
INPUT method takes 1 input, emits 1 ouput and may have an option. INPUT starts on arriving an object to input 1, opens prompt dialog to take an user input in literal syntax of VPGL Object, then emit this input object to output 1. If this method invocation with a option, it will be a prompt message at this dialog.
Malformed user input will cause error.