assertpmux - adds asserts for parallel muxes

yosys> help assertpmux
assertpmux [options] [selection]
This command adds asserts to the design that assert that all parallel muxes
($pmux cells) have a maximum of one of their inputs enable at any time.
do not enforce the pmux condition during the init state
usually the $pmux condition is only checked when the $pmux output
is used by the mux tree it drives. this option will deactivate this
additional constraint and check the $pmux condition always.