Function: sinc
Section: transcendental
C-Name: gsinc
Prototype: Gp
Help: sinc(x): sinc function of x.
Doc: cardinal sine of $x$, i.e. $\sin(x)/x$ if $x\neq 0$, $1$ otherwise.
 Note that this function also allows to compute
 $$(1-\cos(x)) / x^2 = \kbd{sinc}(x/2)^2 / 2$$
 accurately near $x = 0$.
