Variable action`Const`

action: {
    set: (
        attr: ParticleAttribute,
        value: Numlike,
        mode?: SetAttributeMode,
    ) => Action;
    setVar: (name: string, value: Numlike) => Action;
    if_: (cond: Expr, then: Action[], else_?: Action[]) => Action;
} = ...

Constructor helpers for Action nodes. Use action.set(attr, value) for the common overwrite case; pass a third arg for add/multiply/etc. modes.