def subtract(self, es):
workingSet = [self]<EOL>for e in es:<EOL><INDENT>newWorkingSet = []<EOL>for w in workingSet:<EOL><INDENT>newWorkingSet += w.__singleIntersect(e)<EOL><DEDENT>workingSet = newWorkingSet<EOL><DEDENT>return workingSet<EOL>
Subtract the BED elements in es from self.

:param es: a list of BED elements (or anything with chrom, start, end)
:return: a list of BED elements which represent what is left of
         self after the subtraction. This might be an empty list.