anchor.diff:  
     real(kind=real64) pure function atan2_rn(y,x)
       use, intrinsic :: iso_fortran_env, only : real64
  +    use numerical_constants, only : pi, pi2
       real(kind=real64), intent(in) :: x,y
       if(x == 0d0) then
         if(y == 0d0) then
  
positive.code:         atan2_rn=atan2(y,x)
  #endif
        else
          atan2_rn=sign(mb_pi2,y)
        end if
      else
        if(y == 0d0) then
  
negative.code: 
    real(kind=real64) pure function atan2_rz(y,x)
      use, intrinsic :: iso_fortran_env, only : real64
      use numerical_constants, only : pi, pi2
      real(kind=real64), intent(in) :: x,y
      if(x == 0d0) then
        if(y == 0d0) then
  