I'll just point out that the way the subamendment is listed, the (a) and (b) are not part of the substitution wording.
If that helps to clarify it.... I'm sure the officials have something else to add, but the (a) and (b) are the wording of the subamendment. It's that we're saying, (a) we're proposing this substitution, and (b) we're proposing this substitution, but the (a)....