Consider a Python program definedOnString.py$.$ This program takes

two parameters $($P$,$ I$).$ It returns $$yes$$ if P$($I$)$ is defined, and $$no$$ otherwise.

Using an approach similar to figure $3\mathrm{.}9$ and claim $3\mathrm{.}4($page $38),$ prove that

definedOnString.py cannot exist.