def _sanity_check(fh):
<EOL>pos = fh.tell()<EOL>try:<EOL><INDENT>line = fh.readline()<EOL><DEDENT>finally:<EOL><INDENT>fh.seek(pos, <NUM_LIT:0>)<EOL><DEDENT>if not line.startswith('<STR_LIT>'):<EOL><INDENT>raise RuntimeError('<STR_LIT>')<EOL><DEDENT>
Check if the file at least looks like AFM.
If not, raise :exc:`RuntimeError`.