Transfer function notation in gangoffour and in general

[olof3]

In the gangoffour function, the transfer function from d -> y and n -> y are referred to as D and N respectively, (the sensitivity and complementary sensitivity functions are referred to as S and T)

However D and N are typically used to denote the spectrum of d and n.

More suitable names for the transfer functions in the gangoffour would be either

1. PS and CS retaining S and T for sensitivity and complementary sensitivity

or
2) Gyd and Gun while retaining S and T
3) Gyd and Gun while with some transfer functions G?? and G?? for S and T

1. seems like the best approach to me, following Feedback Systems by Åström and Murray

Would perhaps be good to adopt some style guide for the notation throughout the package.