DLAEV2 computes the eigendecomposition of a 2-by-2 symmetric matrix
    [  A   B  ]
    [  B   C  ].
 On return, RT1 is the eigenvalue of larger absolute value, RT2 is the
 eigenvalue of smaller absolute value, and (CS1,SN1) is the unit right
 eigenvector for RT1, giving the decomposition

    [ CS1  SN1 ] [  A   B  ] [ CS1 -SN1 ]  =  [ RT1  0  ]
    [-SN1  CS1 ] [  B   C  ] [ SN1  CS1 ]     [  0  RT2 ].

          A is DOUBLE PRECISION
          The (1,1) element of the 2-by-2 matrix.

          B is DOUBLE PRECISION
          The (1,2) element and the conjugate of the (2,1) element of
          the 2-by-2 matrix.

          C is DOUBLE PRECISION
          The (2,2) element of the 2-by-2 matrix.

          RT1 is DOUBLE PRECISION
          The eigenvalue of larger absolute value.

          RT2 is DOUBLE PRECISION
          The eigenvalue of smaller absolute value.

          CS1 is DOUBLE PRECISION

          SN1 is DOUBLE PRECISION
          The vector (CS1, SN1) is a unit right eigenvector for RT1.

  RT1 is accurate to a few ulps barring over/underflow.

  RT2 may be inaccurate if there is massive cancellation in the
  determinant A*C-B*B; higher precision or correctly rounded or
  correctly truncated arithmetic would be needed to compute RT2
  accurately in all cases.

  CS1 and SN1 are accurate to a few ulps barring over/underflow.

  Overflow is possible only if RT1 is within a factor of 5 of overflow.
  Underflow is harmless if the input data is 0 or exceeds
     underflow_threshold / macheps.