run
Runs or reuses one positional group in the current composition scope.
signature and the active withKeys stack identify the group at its sibling position. A mismatch replaces that group and all following siblings at the drift point. Equal inputs permit reuse while the scope is clean; changed inputs mark the scope dirty. When the body does run, reuseResult may retain the previous result identity after comparing it with the newly calculated value.
Return
the cached, newly calculated, or explicitly reused group result
Parameters
stable structural identity at the current sibling position
value compared with equals to detect explicit input changes
optional predicate returning true to retain the previous result instance
group body, receiving the opaque scope owned by this composer
Type Parameters
type of value cached for the group
Throws
when called outside an active composition attempt