Invisible
Wolfram Kernel
Invisible[expr_]makes an expression invisible, but keeps the space occupied
For example
Framed[Invisible[Integrate[f[x], {x, -\[Pi]/2, \[Pi]/2}]]](*BB[*)((*BB[*)((*TB[*)Integrate[(*|*)f[x](*|*), {(*|*)x(*|*),(*|*)-(*FB[*)(([Pi])(*,*)/(*,*)(2))(*]FB*)(*|*),(*|*)(*FB[*)(([Pi])(*,*)/(*,*)(2))(*]FB*)(*|*)}](*|*)(*1:eJxTTMoPSmNiYGAo5gESnnklqelFiSWpTvkVmYwgURYgEVJUmgoAvmMJeQ==*)(*]TB*))(*,*)(*"1:eJxTTMoPSmNkYGAoZgESHvk5KWlMIB4HkAguqcxJdcqvgIiA5INKc1KLeUBSGfnlzvl5Jal5JcXFrEABt8Sc4lRsCkuKMgv88zzzCkpLEAoBE0ocRw=="*)(*]BB*))(*,*)(*"1:eJxTTMoPSmNkYGAoZgESHvk5KRAeB5BwK0rMTXXKr0hjgskHleakFvMAGcElRZkF/nmeeQWlJcWsILWJOcWpAG8uEdg="*)(*]BB*)