FUNCTION RotationY,  theta
; the angle theta should be in radians


; Note the liberal use of continuation symbols $ to spread the code
; over multiple lines so it is easier to read.
    Ry = [ $
           [ cos(theta),  0,  -sin(theta)], $
           [     0,       1,       0     ], $
           [ sin(theta),  0,   cos(theta)] $
         ]

    RETURN,  Ry
END