A tuple pairing a per-item element placeholder with an optional collection-wide Selection — filtering,
ordering, and pagination. For tabular collections the element is a Projection; relationally, Projection
is the projection (π) and Selection the selection (σ) applied over the collection.
The accepted forms are:
readonly [Union, Selection?] — a union-typed element (Unionkeyed form), optionally followed by a
Selection
readonly [Projection, Selection?] — a tabular projection (Projection), optionally followed by a
Selection
The leading element signals collection cardinality and carries the per-item template; the optional second element
carries collection-wide constraints. Runtime validators accept a one- or two-element tuple and reject arrays of
any other length.
Collection retrieval template.
A tuple pairing a per-item element placeholder with an optional collection-wide Selection — filtering, ordering, and pagination. For tabular collections the element is a Projection; relationally,
Projectionis the projection (π) andSelectionthe selection (σ) applied over the collection.The accepted forms are:
readonly [Union, Selection?]— a union-typed element (Union keyed form), optionally followed by a Selectionreadonly [Placeholder, Selection?]— a per-item Placeholder (a Literal or Reference, or a nested Template), optionally followed by a Selectionreadonly [Projection, Selection?]— a tabular projection (Projection), optionally followed by a SelectionThe leading element signals collection cardinality and carries the per-item template; the optional second element carries collection-wide constraints. Runtime validators accept a one- or two-element tuple and reject arrays of any other length.