Trigonometric reduction formulas

[parisse]

tan(pi/2-b) is well defined, but if you apply the formula that texpands apply, it will involve tan(pi/2) and that's infinity...
Of course it's possible to add a special check for pi/2-b, but that's not so easy if you want to handle all multiples of pi/2, like tan(3*pi/2-b) and of course also tan(b+pi/2), etc.
Unfortunately, I do not have infinite time ressources, I must make choices...