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You’ve got it backwards
&.

The under conjunction, denoted by &. , is like composition. The expression

   (f &. g) x

is defined to mean

   g^:_1 f g x

that is, g is applied to the argument, then f is applied to this result, and finally the verb obverse to g is applied (the obverse is usually the inverse).

For example, to perform a sum scan from right to left,

   sum=.+/
   sumscan=.sum\
   sumscan 1 2 3 4
1 3 6 10
   backwards=.&.|.
   sumscan backwards 1 2 3 4
10 9 7 4