User blog comment:B1mb0w/Growth Rate of the S Function/@comment-5529393-20160712123611/@comment-10262436-20160712125841

My second answer explained how to reduce your example.

Note S(n,T(0),1) is a restricted S Function. In general restricted S functions can only have restricted S functions for their 'a' and 'c' parameters. the b parameter can be generalised.

A generalised S function can contain generalised S functions for any of its a,b or c parameters.

Because S(n,T(0),1) is restricted, then it is a valid 'a' parameter 'parent' to substitue for T(0)