Reductions
Simple explanation
A reduction takes several finished pieces (for example expression, +, term) and glues them into one new node (expression), computing its span from the first and last child.
Technical explanation
parser/reduce.zig builds interior nodes: symbol and production id come from the ReduceRule; byte/point spans come from the outer children; named_child_count and descendant_count aggregate over children; has_error propagates from any tainted child. Pending ERROR leaves whose spans fall inside the new node are spliced into the child list in positional order, which is how skipped text lands inside the smallest enclosing construct. The new node's reuse_state is inherited from its first child — the LR state active where the node starts — enabling later incremental reuse.