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(* Modified for LPM/6 *)(z_axis, arg : direction) : direction;
LOCAL
x_axis : direction;
v : direction;
z : direction;
x_vec : vector;
END_LOCAL;
IF (NOT EXISTS(z_axis)) THEN
RETURN (?) ;
ELSE
z := normalise(z_axis);
IF NOT EXISTS(arg) THEN
IF ((z.direction_ratios <> [1.0,0.0,0.0]) AND
(z.direction_ratios <> [-1.0,0.0,0.0])) THEN
v := dummy_gri || direction([1.0,0.0,0.0]);
ELSE
v := dummy_gri || direction([0.0,1.0,0.0]);
END_IF;
ELSE
IF (arg.dim <> 3) THEN
RETURN (?) ;
END_IF;
IF ((cross_product(arg,z).magnitude) = 0.0) THEN
RETURN (?);
ELSE
v := normalise(arg);
END_IF;
END_IF;
x_vec := scalar_times_vector(dot_product(v, z), z);
x_axis := vector_difference(v, x_vec).orientation;
x_axis := normalise(x_axis);
END_IF;
RETURN(x_axis);
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