#!/usr/bin/env python

def Merge(x,y):
    """ Returns a float list that is the merge of sorted lists x and y.
    
    PreC: x and y are lists of floats that are sorted from small to big.
    """
    # Make copies of x and y so as not to modify these lists in the caller
    u = list(x)
    v = list(y)
    # z will be built up through repeated appending
    z = []
    while len(u)>0 and len(v)>0 :
        if u[0]<= v[0]:
            g = u.pop(0)
        else:
            g = v.pop(0)
        z.append(g)
        print z
    # On eof the lists u and v have been exhauisted.
    # Add whatever is left onto the end of z
    z.extend(u)
    z.extend(v)
    print z
    return z

if __name__ == '__main__':
    
    x = [10,20,30,40,50]
    y = [1,11,12,31]
    print x
    print y
    z = Merge(x,y)