the cell graph: sources at the base, derivations above, edges point to what a cell reads
Last change
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derivations recomputed
Nudge a source cell below to change it. Only the cells that depend on it recompute.
Incr memoizes each derivation by the hash of its value. Change one source and only the affected cone recomputes. A derivation whose recomputed value is unchanged by hash cuts the cascade off, so its dependents stay cached. Exact content comparison, not a written equality.