/*15(a)*/

a=matrix([[1,1,1,1],[0,0,0,0],[0,0,0,0]])
print a
a.rank()
          /*15(b)*/
a=matrix([[1,0,1,0],[0,1,0,1],[0,0,0,0],[0,0,0,0]])
print a
a.rank()
           /*15(c)*/
a=matrix([[1,-1,1,-1],[0,0,0,0],[0,0,0,0]])
print a
a.rank()

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