Bendtorch.OptState.join: def of bendtorch@1.3.0.0
Import: import bendtorch@1.3.0.0/main.bend as Bendtorch
Lines 89-96 of bendtorch@1.3.0.0/main.bend: https://jend.leeeet.dev/read.txt?path=bendtorch@1.3.0.0/main.bend&start_line=89&end_line=96

import Base

# Pair the buffers present on both sides. Internal callers split one state, then step both children with the same
# optimizer, so the variants agree. No buffer is ever allocated to fill an absent side.
def OptState.join(-A: Type, -B: Type, a: OptState<A>, b: OptState<B>) -> OptState<Two<A, B>>:
  match a b:
    case OneMom{x} OneMom{y}: OneMom{Two{x, y}}
    case TwoMom{x, u} TwoMom{y, v}: TwoMom{Two{x, y}, Two{u, v}}
    case Packed{x} Packed{y}: Packed{PNode{x, y}}
    case _ _: NoMom{}

Uses (3):
  https://jend.leeeet.dev/definition.txt?path=bendtorch@1.3.0.0/main.bend&name=OptState
  https://jend.leeeet.dev/definition.txt?path=bendtorch@1.3.0.0/main.bend&name=PTree
  https://jend.leeeet.dev/definition.txt?path=bendtorch@1.3.0.0/main.bend&name=Two