Example 2 (p. 732)

> F := [ -y/2, x/2 ];

F := [-1/2*y, 1/2*x]

> fieldplot( F, x=-2..2, y=-2..2 );

[Maple Plot]

>

> r := [ x, y ];

r := [x, y]

>

> innerprod( r, F );

0

>