"expects valid input: schemas is a named list"
"expects valid input: schema elements correct lengths"
"expects valid input: schema elements contain characters of valid lengths"
"expects valid input: key attributes are in relation"
"orders key attributes with respect to order in attrs_order"
"orders attributes with respect to appearance in keys, then by attrs_order"
"is subsetted to a valid relation schema, follows usual subsetting rules..."
"expects a relation_schema value for subset re-assignment"
"^value must also be a relation_schema object$"
"is made unique to a valid relation schema"
"is composed of its attrs(), keys(), names(), and attrs_order()"
"is created with logical attribute classes"

relation_schema / concatenates without losing attribute orderings, if consistent