fix problem that only one box is produced when using fx, fy or fz.

[after12am]

fix https://github.com/after12am/eisenscript/issues/73#issuecomment-670698449

ex)

2 * { fx } 1 * { x 5 } box
2 * { fy } 2 * { fx } 1 * { x 1 y 3 } box