Write a function that tells you which of its lines has been removed
Ruby

Eschewing magic numbers since it's not code golf.

def f
  r=2^3^4^5
  defined?(r) ? r^=2 : r=1^3^4^5
  r^=3
  r^=4
  r^=5
end


Each line strips its own number out of 1^2^3^4^5. It's Ruby, so the last line defines the return value.