Register motif under renaming

A compiler pass writes a trace where each step is named by the register it wrote, one lowercase letter per step. A clone detector hunts for a motif that turns up again under a consistent renaming of registers, since the shape of the reuse matters and the names do not. A window of the trace matches the motif when there is a one to one pairing between the letters of the motif and the letters of the window that turns one into the other, so the same motif letter always pairs with the same window letter and two motif letters never share a window letter. The motif ab therefore matches xy and yx but never zz. Given trace and motif, return the start index of every window that matches, ascending. Windows may overlap. Return an empty list when the motif is empty or longer than the trace.