df-sbt

Definition.

Arguments:

a (st), x (sv), NIL (sv), NIL (sv), y (sv),

Distinct variable conditions:

(y, a), (y, x), (y, NIL), (y, NIL),

Assertions:

(stopsignaxNILNILleftrightarrowforally((yinNIL)leftrightarrow[a/x](yinNIL)))