introduce notation for the magic return variable name

theories/lang/notation.v

theories/typing/examples/init_prod.v

theories/typing/lib/cell.v

theories/typing/lib/fake_shared_box.v

theories/typing/lib/option.v

theories/typing/lib/rc.v

theories/typing/lib/refcell/ref_code.v

theories/typing/lib/refcell/refcell_code.v

theories/typing/lib/refcell/refmut_code.v

theories/typing/lib/rwlock/rwlock_code.v

theories/typing/lib/rwlock/rwlockreadguard_code.v

theories/typing/lib/rwlock/rwlockwriteguard_code.v

theories/typing/lib/spawn.v