code: def Append(self, x, p):
          
          self.xs.append(x)
          self.ps.append(p)
docs: Add an (x, p) pair to the end of this CDF.
  
          Note: this us normally used to build a CDF from scratch, not
          to modify existing CDFs.  It is up to the caller to make sure
          that the result is a legal CDF.