class SourceMap::Map

def +(other)
  mappings = @mappings.dup
  offset   = @mappings.any? ? @mappings.last.generated.line+1 : 0
  other.each do |m|
    mappings << Mapping.new(
      m.source, m.generated + offset,
      m.original, m.name
    )
  end
  self.class.new(mappings, other.filename)
end