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The notion of aggregate serves to model any data structure aggregate, designating any collection of objects, with heterogeneous or homogeneous members, with a finite or infinite number of members, explicitly or implicitly represented. An aggregate can in principle represent everything from a string of characters to abstract sets such as “the set of x satisfying relation r(x)” An attribute finiteAggregate is used to assert that a domain has a finite number of elements.

#: % -> NonNegativeInteger if % has finiteAggregate
\#(u) returns the number of elements in u.
copy: % -> %
copy(u) returns a top-level (non-recursive) copy of u. Note: for collections, copy(u) = [x for x in u].
empty: () -> %
empty()$D creates an aggregate of type D with 0 elements. Note: The $D can be dropped if understood by context, e.g. u: D := empty().
empty?: % -> Boolean
empty?(u) tests if u has 0 elements.
eq?: (%, %) -> Boolean
eq?(u, v) tests if u and v are same objects.
less?: (%, NonNegativeInteger) -> Boolean
less?(u, n) tests if u has less than n elements.
more?: (%, NonNegativeInteger) -> Boolean
more?(u, n) tests if u has more than n elements.
sample: %
sample yields a value of type %
size?: (%, NonNegativeInteger) -> Boolean
size?(u, n) tests if u has exactly n elements.