some technical information about various PDF interfaces
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- PYTHIA 6.2 parton numbering scheme:

   -6  -5  -4  -3  -2  -1   0  1   2   3  4  5  6
   t~  b~  c~  s~  u~  d~   g  d   u   s  c  b  t

- PDFLIB: 
  - takes transferred momentum (Q) as an input
  - doesn't have a parton numbering scheme visible outside
  - returns x*f(x)

- CTEQ6:
  - takes Q as a parameter
  - return f(x)
  - numbering scheme: unlike in PYTHIA u=1 and d=2

C   Iparton  is the parton label (5, 4, 3, 2, 1, 0, -1, ......, -5)
C                            for (b, c, s, d, u, g, u_bar, ..., b_bar),

- MRST:
  - takes Q as parameter
  - returns x*f(x)
  - doesn't have a numbering scheme visible outside
  - interface doesn't have TOP parameter and GLU appears at a place 
    where you'd naively expect fo find TOP.