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Z←{L} fg R applies the function g then f and then g⍣¯1 to or between the arguments.
L is an optional array.
R is an array.
f is an arbitrary ambivalent function.
g is an arbitrary monadic function with an inverse.


The concept of the Dual operator was described in Ken Iverson's paper on Operators and Functions.


This operator is implemented via the anonymous function:

{⍺←⍣0 ⋄ ⍵⍵⍣¯1 (⍵⍵ ⍺) ⍺⍺ ⍵⍵ ⍵}