SLAEV2 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 REAL
          The (1,1) element of the 2-by-2 matrix.

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

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

          RT1 is REAL
          The eigenvalue of larger absolute value.

          RT2 is REAL
          The eigenvalue of smaller absolute value.

          CS1 is REAL

          SN1 is REAL
          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.