def read_fortran_file (filename):
        """ Returns a generator that yields all numbers in the Fortran file as float, one by one"""
        import os.path
        if not(os.path.exists(filename)) : raise IOError, "File %s does not exist."%filename
        for line in open(filename,'r') :
            for x in line.replace('D','E').split() :
                yield string.atof(x)

    def constr_Sigma_real_axis(self, filename, hdf=True, hdf_dataset='SigmaReFreq',n_om=0,orb=0, tol_mesh=1e-6):
        """Uses Data from files to construct Sigma (or GF)  on the real axis."""

        if not hdf: # then read sigma from text files

            # first get the mesh out of any one of the files:
            bl = self.gf_struct_solver[orb].items()[0][0] # block name
            ol = self.gf_struct_solver[orb].items()[0][1] # list of orbital indices
            if (len(ol)==1): # if blocks are of size one
                Fname = filename+'_'+bl+'.dat'
            else:
                Fname = filename+'_'+bl+'/'+str(ol[0])+'_'+str(ol[0])+'.dat'

            R = read_fortran_file(Fname)
            mesh = numpy.zeros([n_om],numpy.float_)
            try:
                for i in xrange(n_om):
                    mesh[i] = R.next()
                    sk = R.next()
                    sk = R.next()

            except StopIteration : # a more explicit error if the file is corrupted.
                raise "SumkLDA.read_Sigma_ME : reading mesh failed!"
            R.close()

            # check whether the mesh is uniform
            bin = (mesh[n_om-1]-mesh[0])/(n_om-1)
            for i in xrange(n_om):
                assert abs(i*bin+mesh[0]-mesh[i]) < tol_mesh, 'constr_Sigma_ME: real-axis mesh is non-uniform!'

            # construct Sigma
            a_list = [a for a,al in self.gf_struct_solver[orb].iteritems()]
            glist = lambda : [ GfReFreq(indices = al, window=(mesh[0],mesh[n_om-1]),n_points=n_om) for a,al in self.gf_struct_solver[orb].iteritems()]
            SigmaME = BlockGf(name_list = a_list, block_list = glist(),make_copies=False)

            #read Sigma
            for i,g in SigmaME:
                mesh=[w for w in g.mesh]
                for iL in g.indices:
                    for iR in g.indices:
                        if (len(g.indices) == 1):
                            Fname = filename+'_%s'%(i)+'.dat'
                        else:
                            Fname = 'SigmaME_'+'%s'%(i)+'_%s'%(iL)+'_%s'%(iR)+'.dat'
                        R = read_fortran_file(Fname)
                        try:
                            for iom in xrange(n_om):
                                sk = R.next()
                                rsig = R.next()
                                isig = R.next()
                                g.data[iom,iL,iR]=rsig+1j*isig
                        except StopIteration : # a more explicit error if the file is corrupted.
                            raise "SumkLDA.read_Sigma_ME : reading Sigma from file failed!"
                        R.close()


        else: # read sigma from hdf

            omega_min=0.0
            omega_max=0.0
            n_om=0
            if (mpi.is_master_node()):
                ar = HDFArchive(filename)
                SigmaME = ar[hdf_dataset]
                del ar

            #OTHER SOLUTION FIXME
            #else:
            #    SigmaME=0
            #SigmaME = mpi.broadcast..

                # we need some parameters to construct Sigma on other nodes
                omega_min=SigmaME.mesh.omega_min
                omega_max=SigmaME.mesh.omega_max
                n_om=len(SigmaME.mesh)
            omega_min=mpi.bcast(omega_min)
            omega_max=mpi.bcast(omega_max)
            n_om=mpi.bcast(n_om)
            mpi.barrier()
            # construct Sigma on other nodes
            if (not mpi.is_master_node()):
                a_list = [a for a,al in self.gf_struct_solver[orb].iteritems()]
                glist = lambda : [ GfReFreq(indices = al, window=(omega_min,omega_max),n_points=n_om) for a,al in self.gf_struct_solver[orb].iteritems()]
                SigmaME = BlockGf(name_list = a_list, block_list = glist(),make_copies=False)
            # pass SigmaME to other nodes
            SigmaME = mpi.bcast(SigmaME)
            mpi.barrier()

        SigmaME.note='ReFreq'

        return SigmaME