exact computation of nj-symbols
The value of an nj-symbol always is plus or minus
the square root of a non-negative rational number.
The square can be computed exactly.
This is what procedures exact-3j-symbol, exact-6j-symbol and exact-9j-symbol do.
They return plus or minus the exact square of the value of the nj-symbol.
The procedures follow formulas C.21, C.36 and C.41 as found in appendix C of:
Quantum Mechanics, Volume II, Albert Messiah, North-Holland publishing company.
Procedures 3j-symbol, 6j-symbol and 9j-symbol return the value proper of an nj-symbol,