Added warning about infinite loops in for-from-by statement.
Added warning about infinite loops in for-from-by statement.

One rarely, if ever, wants the [step] argument to be negative.

da-woods/cython,fabianrost84/cython,achernet/cython,JelleZijlstra/cython,dahebolangkuan/cython,slonik-az/cython,hhsprings/cython,da-woods/cython,mrGeen/cython,hhsprings/cython,madjar/cython,hpfem/cython,roxyboy/cython,scoder/cython,fperez/cython,marscher/cython,dahebolangkuan/cython,larsmans/cython,c-blake/cython,hickford/cython,mcanthony/cython,acrispin/cython,encukou/cython,fabianrost84/cython,c-blake/cython,marscher/cython,slonik-az/cython,larsmans/cython,c-blake/cython,mrGeen/cython,hickford/cython,hickford/cython,dahebolangkuan/cython,hpfem/cython,da-woods/cython,hpfem/cython,ABcDexter/cython,cython/cython,ABcDexter/cython,hhsprings/cython,mrGeen/cython,mrGeen/cython,mrGeen/cython,ChristopherHogan/cython,bzzzz/cython,hickford/cython,bzzzz/cython,hpfem/cython,rguillebert/CythonCTypesBackend,ABcDexter/cython,rguillebert/CythonCTypesBackend,fperez/cython,andreasvc/cython,acrispin/cython,acrispin/cython,hhsprings/cython,hhsprings/cython,scoder/cython,fabianrost84/cython,achernet/cython,roxyboy/cython,ChristopherHogan/cython,madjar/cython,madjar/cython,andreasvc/cython,JelleZijlstra/cython,marscher/cython,c-blake/cython,roxyboy/cython,da-woods/cython,JelleZijlstra/cython,achernet/cython,fperez/cython,scoder/cython,bzzzz/cython,larsmans/cython,cython/cython,slonik-az/cython,encukou/cython,slonik-az/cython,dahebolangkuan/cython,bzzzz/cython,encukou/cython,JelleZijlstra/cython,acrispin/cython,scoder/cython,madjar/cython,dahebolangkuan/cython,roxyboy/cython,andreasvc/cython,marscher/cython,cython/cython,ABcDexter/cython,cython/cython,mcanthony/cython,c-blake/cython,slonik-az/cython,mcanthony/cython,JelleZijlstra/cython,fperez/cython,roxyboy/cython,fabianrost84/cython,larsmans/cython,madjar/cython,mcanthony/cython,fperez/cython,hickford/cython,achernet/cython,ChristopherHogan/cython,encukou/cython,fabianrost84/cython,rguillebert/CythonCTypesBackend,marscher/cython,larsmans/cython,andreasvc/cython,ABcDexter/cython,acrispin/cython,mcanthony/cython,hpfem/cython,andreasvc/cython,achernet/cython,encukou/cython,rguillebert/CythonCTypesBackend