True and false and true Challenge is to write a function/program which outputs a truthy or falsey value alternately each time you call it. No input.

It can start with either true or false.
Jelly, 3 bytes

®¬©


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How it works

®¬©  Main link. No arguments.

®    Retrieve the value stored in the register.
 ¬   Negate it.
  ©  Copy the result to the register.