assert_is_function(x, severity = getOption("assertive.severity", "stop"))
assert_is_primitive(x, severity = getOption("assertive.severity", "stop"))
assert_is_stepfun(x, severity = getOption("assertive.severity", "stop"))
is_function(x, .xname = get_name_in_parent(x))
is_primitive(x, .xname = get_name_in_parent(x))
is_stepfun(x, .xname = get_name_in_parent(x))