Planned@ At r←[x]({Fn}@m)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 cells of y that appear in m with elements provided by F or n: if n is provided, it must have the same shape as y or one less rank and same trailing shape, and corresponding elements in n are used from m, or the whole of n if it is of lower rank; if F is provided, it is called with the cells of y matching m and optionally x, and the result is used like n.