The function subgrel is used as a procedure and an unevaluated procedure call. As a procedure, subgrel checks its arguments and then either exits with an error or returns the unevaluated subgrel call.

The first argument is a set of equations or words on the generators of the parent group. The left side of each equation is the name of a subgroup generator; the right side is a word in the group generators (see grelgroup) representing the subgroup generator. If only a word is specified, then a name for the generator is produced automatically.

Error, (in subgrel) generator [a, b, c] contains a syllable `c' that is not a generator, or the inverse of a generator, of the parent group