Hello Rick,

As usual, useful, concise, material.

But another typo I believe.
this: input x1-x4 c $ xray; /* x1-x4 is shorthand for INPUT c1 c2 c3 c4; */
should likely be this: input x1-x4 c $ xray; /* x1-x4 is shorthand for INPUT x1 x2 x3 x4; */