  We show that inverse square singularities can be treated as boundary
conditions containing rational Herglotz--Nevanlinna functions of the eigenvalue
parameter with "a negative number of poles". More precisely, we treat in a
unified manner one-dimensional Schr\"{o}dinger operators with either an inverse
square singularity or a boundary condition containing a rational
Herglotz--Nevanlinna function of the eigenvalue parameter at each endpoint, and
define Darboux-type transformations between such operators. These
transformations allow one, in particular, to transfer almost any spectral
result from boundary value problems with eigenparameter dependent boundary
conditions to those with inverse square singularities, and vice versa.
