according to the
elements of
and attempts to combine them
using the particular list of rules called rulesCollect.
In the noncommutative case, the Taylor
expansion, and hence the collect function, is not uniquely
specified. This collect function often collects too much and
while mathematically correct is often stronger than you want.
For example, 
will
factor out of terms where it appears both linearly a
quadratically thus mixing orders.
is an algebraic expression.
is a list of variables.