$: is a proxy that assumes result of 片語in
which it occurs, 片語being terminated on 左by a copula
or by completion of 句子. 例如:
1:`(] * $:@<:)@.* 5
在foregoing expression, 議程(@.)
chooses 動詞 ] * $:@<: as long as 引數
(reduced by one each time by application of 減值)
remains non-zero. When 引數becomes zero, result of
右引數of @. is zero,與constant
函數 1: is chosen.
If $:@ were omitted from expression,
it would execute once only as follows:
1:`(] * <:)@.* 5
inclusion of self-reference ensures that entire
函數 is re-executed after 減值ing 引數.