# The name

**croak** stands for *Complete Retrieval by Optimisation and Adjoint Kernels*.

*Complete* refers to what is retrieved and what is modelled: the full complex
field, amplitude *and* phase, through a forward model that includes the
measurement physics (propagation in the nonlinear medium, beam geometry and
collection) rather than a thin-medium idealisation of it. *Adjoint kernels*
names the machinery: each interaction (SHG, SD, PG) contributes a kernel to the
forward map, and each carries a matching hand-derived
[Wirtinger adjoint](../explanation/gradients.md) that yields exact gradients for
the cost of one extra forward pass.

## Why it used to be called toad

For as long as it was an internal tool the package was called **toad** — *Trace
Optimisation through Automatic Differentiation*. That acronym described what was
then the headline feature: the forward model is differentiable end to end, so a
JAX twin of every solver can be built and used to check the analytic gradients
against automatic differentiation.

The name could not be kept for the public release, because `toad` was already
taken on the Python Package Index. Rather than decorate it — `toad-frog`,
`pytoad`, and similar — the package was renamed outright.

Automatic differentiation is, if anything, more central now than when it named
the package: geometric smearing, the focal and collection models, the fitted
medium thickness and delay-zero, the B-spline phase basis and the Laplace
covariance all depend on it. What changed is that it no longer covers the whole
package, which grew a hand-derived Wirtinger adjoint for the core model, a
complete measurement-to-result workflow and uncertainty quantification.
*Adjoint kernels* covers both routes: each interaction contributes a kernel to
the forward map, and its derivative is available either in closed form or
through AD.

## The mascot

The mascot was not renamed. The wizard's icon is still a toad, which — given what
toads are known for — remains approximately correct.

```{note}
Nothing in the public API ever carried the old name, so there is no compatibility
shim to be aware of: the package is imported as `croak`, the console scripts are
`croak` and `croak-gui`, and saved result files record a `croak_version` header.
Files written by the pre-release `toad` package are not read by this one.
```