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

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

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

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

$\mathrm{p}(\mathrm{p}$

$\mathrm{p}=\mathrm{p}$

There should be no horizontal gap around the middle rectangles:

$\mathrm{p}(\mathrm{p}$

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

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

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

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

$\mathrm{p}\&\mathrm{p}$