--Copyright The Numerical Algorithms Group Limited 1991.

sinCosExpand := rule
  sin(-x)    == - sin(x)
  cos(-x)    == cos(x)
  sin(x + y) == sin(x) * cos(y) + sin(y) * cos(x)
  cos(x + y) == cos(x) * cos(y) - sin(x) * sin(y)
  sin((n | integer? n and n > 1) * x) ==_
       sin(x) * cos((n-1)*x) + sin((n-1)*x) * cos(x)
  cos((n | integer? n and n > 1) * x) ==_
       cos(x) * cos((n-1)*x) - sin(x) * sin((n-1)*x)
