⍳ Index Of r←x⍳y

r is the index of corresponding cells of y in the major cells of x. If a cell of y does not appear in the cells of x, the corresponding element of r is the tally of x. Supports the origin extra argument, which offsets the result by the origin; supports the tolerance extra argument for the comparisons; supports the backward extra argument, which returns the last index; supports the fill extra argument, which is returned when a cell is not found.