A __p__remature __ret__urn to __i__mmediate e__x__ecution. That is, a sentence which produces no result, in the same way 3 : 'throw.' 0 produces no result.
A pretix is visually indistinguishable from a valid-but-empty result. For example, here's a normal, empty, result:
i. 0 0 }}} and here's a pretix:
3 : 'throw.' 0
To distinguish the two, you may use an assignment. If the name remains unassigned after the sentence is executed (without error), you've got a pretix:
e=. i. 0 0 f=. 3 : 'throw.' 0 e NB. e is defined f NB. But f is not (value error produced) |value error: f NB. e is a noun, f is undefined. 4!:0 ;:'e f' 0 _1