Function tests whether x is a valid rule object, i.e., whether
it is list a list and inherits from the rmake.rule S3 class. Instances
of rule represent an atomic building unit, i.e. a command that
has to be executed, which optionally depends on some files or other
rules -- see rule() for more details.
is.rule(x)An argument to be tested
TRUE if x is a valid rule object and FALSE otherwise.