sub norm{ my $v1,$v2,$r; if( defined $v2 ){ $v1 = $v2; undef $v2; }else{ do{ $v1=rand(2)-1; $v2=rand(2)-1; $r = $v1*$v1+$v2*$v2; }while( $r >= 1 || $r == 0 ); $r = sqrt(-2*(log $r)/$r); $v1 *= $r; $v2 *= $r; } return $v1; }