fn merge_source_maps(
lhs: &HashMap<VariableId, WriteSources>,
rhs: &HashMap<VariableId, WriteSources>,
) -> HashMap<VariableId, WriteSources>fn merge_source_maps(
lhs: &HashMap<VariableId, WriteSources>,
rhs: &HashMap<VariableId, WriteSources>,
) -> HashMap<VariableId, WriteSources>