Separate the covariates from a 'formula' with a cluster effect from the ones with an identical effect in each cluster if provided.

1 |

`formula` |
A symbolic description of the model. In
the parametric case we write for example |

Given a 'formula' of the form ```
Y ~ clust(T1 + T2 +
...) + pop(X1 + X2 + ...)
```

or just ```
Y ~ clust(T1 +
T2 + ...)
```

, it returns a list of two or one 'formula' of
the form `Y ~ T1 + T2 + ...`

and ```
~ X1 + X2 +
...
```

if provided.

The first element of the list
correspond to the covariates with a different effects
corresponding to the cluster, the 2nd correspond to
covariates having an identical effect in each cluster.

In the non-parametric case only **one** covariate is
allowed.

A list of 1 or 2 [formula].

Meant to be used internally.

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