{HEAD ~ An inner housing} defines {TAIL ~ a cylindrical outer wall} and, together with the substantially cylindrical inner wall, forms a substantially cylindrical ring gap 
defines
{ HEAD ~ An inner housing } defines { TAIL ~ a cylindrical outer wall } and , together with the substantially cylindrical inner wall , forms a substantially cylindrical ring gap
HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH