Return the symbol used in the bytecode to represent the `library_name`.

def solidity_library_symbol(library_name):
    """ Return the symbol used in the bytecode to represent the `library_name`. """
    # the symbol is always 40 characters in length with the minimum of two
    # leading and trailing underscores
    length = min(len(library_name), 36)

    library_piece = library_name[:length]
    hold_piece = '_' * (36 - length)

    return '__{library}{hold}__'.format(
        library=library_piece,
        hold=hold_piece,
    )