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Self Reference $:  _ _ _  

$: is a proxy that assumes the result of the phrase in which it occurs, the phrase being terminated on the left by a copula or by the completion of the sentence. For example:
   1:`(] * $:@<:)@.* 5
120
In the foregoing expression, the agenda (@.) chooses the verb ] * $:@<: as long as the argument (reduced by one each time by the application of the decrement) remains non-zero. When the argument becomes zero, the result of the right argument of @. is zero, and the constant function 1: is chosen.

If $:@ were omitted from the expression, it would execute once only as follows:
   1:`(] * <:)@.* 5
20
The inclusion of self-reference ensures that the entire function is re-executed after decrementing the argument.
 




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