Examples

R ??pred S = ?{ (r,s) | r ? R, s ? S, pred }

flatten(R) = ?{ s | r ? R, s ? r }

R ? S = ?{ r | r ? R, r ? S }

size(R) = +{ 1 | r ? R }

e ? R = ?{ r = e | r ? R }

R ? S = ?{?{ r = s | s ? S } | r ? R }

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Here are some examples of monoid comprehensions. Monoid comprehensions capture the nested relation algebra for multiple collection types, plus aggregates and quantification.