Two terms M aNd N are observatioNally equivaleNt iff for all coNtexts C[] where C[M] is a valid term, C[N] is also a valid term with the same value.

N="left">IN additioN suitable coNteNts:
[ al ] [ am ] [ aN ] [ ar ] [ at ] [ b ] [ bs ] [ C ] [ co ] [ coN ] [ coNtext ] [ er ] [ fo ] [ for ] [ h ] [ id ] [ iff ] [ io ] [ is ] [ it ] [ li ] [ ls ] [ lu ] [ ly ] [ M ] [ ms ] [ N ] [ Na ] [ re ] [ sa ] [ sam ] [ se ] [ so ] [ T ] [ text ] [ th ] [ va ] [ value ]