There should be 5/18em horizontal gaps around the middle rectangles:

$\mathrm{p}<span>X</span>\mathrm{p}$

$\mathrm{p}X<span></span>\mathrm{p}$

$\mathrm{p}<span></span>X\mathrm{p}$

$\mathrm{p}X\mathrm{p}$

$\mathrm{p}XX\mathrm{p}$

There should be no horizontal gap around the middle rectangles:

$\mathrm{p}X\mathrm{p}$

$\mathrm{p}<span></span>X\mathrm{p}$

$\mathrm{p}<!--\; COMMENT\; -->X\mathrm{p}$

There should be 4/18em horizontal gaps around the middle rectangles:

$\mathrm{p}XX\mathrm{p}$

$\mathrm{p}XX\mathrm{p}$