Planned@ At r←[x]({Fn}@{Gm})y
This primitive or feature is planned, that is, it doesn't appear in the most recent released implementation of TinyAPL. Details may change before it is added!
If x is provided, the left operand must be a function.
r is the result of substituting elements indicated by G or m with elements provided by F or n: if m is provided, it is a list of indices of major cells of y to be substituted; if G is provided, it is a function that returns a mask of the same shape as y; if n is provided, it is an array whose items will be used to replace the selected ones; if F is provided, it is a function that is applied to the selected items (as a whole), optionally with left argument x.