similar
substitutable
identical
interchangeable
akin
analogous
applicable
coincident
extinct
impregnable
inaccessible
incompatible
indistinguishable
inseparable
insurmountable
intact
invulnerable
irrelevant
negligible
nonexistent
unable
unchanged
undetectable
unheard
uninhabitable
valueless
anonymous
autonomous
antonymous
nonsynonymous