Function: prod
Section: sums
C-Name: produit
Prototype: V=GGEDG
Help: prod(X=a,b,expr,{x=1}): x times the product (X runs from a to b) of
 expression.
Doc: product of expression
 \var{expr}, initialized at $x$, the formal parameter $X$ going from $a$ to
 $b$. As for \kbd{sum}, the main purpose of the initialization parameter $x$
 is to force the type of the operations being performed. For example if it is
 set equal to the integer 1, operations will start being done exactly. If it
 is set equal to the real $1.$, they will be done using real numbers having
 the default precision. If it is set equal to the power series $1+O(X^k)$ for
 a certain $k$, they will be done using power series of precision at most $k$.
 These are the three most common initializations.

 \noindent As an extreme example, compare

 \bprog
 ? prod(i=1, 100, 1 - X^i);  \\@com this has degree $5050$ !!
 time = 128 ms.
 ? prod(i=1, 100, 1 - X^i, 1 + O(X^101))
 time = 8 ms.
 %2 = 1 - X - X^2 + X^5 + X^7 - X^12 - X^15 + X^22 + X^26 - X^35 - X^40 + \
 X^51 + X^57 - X^70 - X^77 + X^92 + X^100 + O(X^101)
 @eprog\noindent
 Of course, in  this specific case, it is faster to use \tet{eta},
 which is computed using Euler's formula.
 \bprog
 ? prod(i=1, 1000, 1 - X^i, 1 + O(X^1001));
 time = 589 ms.
 ? \ps1000
 seriesprecision = 1000 significant terms
 ? eta(X) - %
 time = 8ms.
 %4 = O(X^1001)
 @eprog

 \synt{produit}{GEN a, GEN b, char *expr, GEN x}.
