Description:
GrabIndeterminants[L] returns the
indeterminates found in the list of (noncommutative)
expressions or rules 
. For example,
GrabIndeterminants[
{ x**Inv[x]**x + Tp[Inv[x+a]],
3 + 4 Inv[a]**b**Inv[a] + x }]
returns
{ x, Inv[x], Tp[Inv[x+a]], Inv[a], b }.
