def get_structure(self, ip):
"""get_structure(ip) --> Get a copy of specified phase data.
ip -- index of existing PdfFit phase starting from 1
Return Structure object from diffpy.Structure.
Raise pdffit2.unassignedError if phase ip is undefined.
"""
from diffpy.Structure import PDFFitStructure
s = self.save_struct_string(ip)
stru = PDFFitStructure()
stru.readStr(s, 'pdffit')
return stru
