Function psubs() substitutes one or more player names, replacing
player from[i] with to[i]. If argument to is
missing, all players are substituted, the second argument taken to be
the replacement: interpret psubs(H,vec) as
psubs(H,from=pnames(H),to=vec).
Compare pnames<-(), which can only add players, or reorder
existing players.
Function psubs_single() is a low-level helper function that takes
a single player and its substitute; it is not intended for direct use.