def new_evidence_file(network_filename, variable, value):
    """Creates a new evidence file for a given goal variable-value pair (and
    returns its filename)."""
    new_filename = network_filename + '.inst'
    with open(new_filename, 'w') as evidence_file:
        evidence_file.write(
            '<?xml version="1.0" encoding="UTF-8"?>\n<instantiation>' +
            '<inst id="{}" value="{}"/>'.format(variable, value) +
            '</instantiation>')
    return new_filename