an optional set of values which specify the desired T-terms.
E.g. x = c(0,1,2) would return T0, T1, and T2 and so on. Negative values of x will
cause the function to stop. Values of x which exceed those observed will be assigned
a value of zero. The return values will be returned in ascending order regardless of
the order of x (although I suppose I could preserve the order if someone really cares).