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   Integrand for pointing budgets.
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   Form:
   y = FPB( theta, rho, sig1, sig2, f )
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   ------
   Inputs
   ------
   theta          (1,:)   Angle
   rho            (1,1)   Correlation
   sig1           (1,1)   Standard deviation 
   sig2           (1,1)   Standard deviation
   f              (1,1)   Coefficient of exponent term

   -------
   Outputs
   -------
   y              (1,:)   Output

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