PropCheck.implies

You're seeing just the macro implies, go back to PropCheck module for more information.
Link to this macro

implies(precondition, list)

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A property that is only tested if a condition is true.

This wrapper only makes sense when in the scope of at least one forall. The precondition field must be a boolean expression or a statement block that returns a boolean. If the precondition evaluates to false for the variable instances produced in the enclosing forall wrappers, the test case is rejected (it doesn't count as a failing test case), and PropCheck starts over with a new random test case. Also, in verbose mode, an x is printed on screen.

iex> use PropCheck
iex> require Integer
iex> quickcheck(
...> forall n <- nat() do
...>    implies rem(n, 2) == 0, do: Integer.is_even n
...> end
...>)
true