override define \s :=
$() $()
endef

ifndef $(\s)
override $(\s) :=
else
$(error Defined special variable '$(\s)': reserved for internal use)
endif

override define dirname
$(patsubst %/,%,$(dir $(patsubst %/,%,$1)))
endef

override define prefix_1
$(if $(or $\
$(patsubst $(abspath $3)%,,$(abspath $1)),$\
$(patsubst $(abspath $3)%,,$(abspath $2))),$\
$(strip $(call prefix_1,$1,$2,$(call dirname,$3))),$\
$(strip $(abspath $3)))
endef

override define prefix
$(call prefix_1,$1,$2,$1)
endef

override define relpath_1
$(patsubst /%,%,$(subst $(\s),/,$(patsubst %,..,$(subst /,$(\s),$\
$(patsubst $3%,%,$(abspath $2)))))$\
$(patsubst $3%,%,$(abspath $1)))
endef

override define relpath
$(call relpath_1,$1,$2,$(call prefix,$1,$2))
endef