pi – Cal­cu­late pi

Gen­er­ates pi, us­ing the for­mula:
     Pi=16*arc­tan(1/5)-4*arc­tan(1/239)
and leaves the re­sult in an ar­ray \xr, print­ing what it’s cal­cu­lated as it goes along.
The num­ber of dig­its you can com­pute de­pends on your im­ple­men­ta­tion of TeX. The last digit may be wrong, and if it's 0 or 9, the penul­ti­mate digit may also be wrong.