This checks whether residues in reslist exist in the ATOM lines.
           It returns a list of the residues in reslist which did exist.

def CheckForPresenceOf(self, reslist):
        '''This checks whether residues in reslist exist in the ATOM lines.
           It returns a list of the residues in reslist which did exist.'''
        if type(reslist) == type(""):
            reslist = [reslist]

        foundRes = {}
        for line in self.lines:
            resname = line[17:20]
            if line[0:4] == "ATOM":
                if resname in reslist:
                    foundRes[resname] = True

        return foundRes.keys()