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Z←L~R returns a subset of L consisting of those elements in L which do not occur in R.
L is a scalar or vector, else signal RANK ERROR.
R is an arbitrary array.
Z is a vector identical to (~L∊R)/L.
The result is sensitive to ⎕CT.

This feature is implemented by an internal magic function:

    ∇ Z←L #DydTilde R
[1]   Z←(~L∊R)/L