n three 
    dimensions, e.g. <a href="jmat3.html">JMAT3</a>(PHI,3) returns
 
        J=[COS(PHI) -SIN(PHI) 0;
           SIN(PHI)  COS(PHI) 0;
              0        0      1]
 
    such that J*X rotates the column-vector X by PHI radians
    counterclockwise in the XY plane.
 
    If LENGTH(PHI)>1, then J will have dimension 3 x 3 x SIZE(PHI).
    N may either be a scalar or an array of the same length as PHI.
 
    See also <a href="vectmult.html">VECTMULT</a>, <a href="jmat2.html">JMAT2</a>.
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