So far, two specials are implemented. `concurrent`

is used when
the goal is to estimate a concurrent effect of the TDC. `cumulative`

is used when the goal is to estimate a cumulative effect of the TDC. These
should usually not be called directly but rather as part of the `formula`

argument to `as_ped`

.
See the vignette on data transformation
for details.

```
cumulative(..., tz_var, ll_fun = function(t, tz) t >= tz, suffix = NULL)
concurrent(..., tz_var, lag = 0, suffix = NULL)
has_special(formula, special = "cumulative")
```

- ...
For

`concurrent`

variables that will be transformed to covariate matrices. The number of columns of each covariate depends on`tz`

. Usually, elements that will be specified here are`time`

(which should be the name of the time-variable used on the LHS of the formula argument to`as_ped`

),`tz`

which is the variable containing information on the times at which the TDC was observed (can be wrapped in`latency`

) and the TDCs that share the same`tz`

and Lag-lead window (`ll_fun`

).- tz_var
The name of the variable that stores information on the times at which the TDCs specified in this term where observed.

- ll_fun
Function that specifies how the lag-lead matrix should be constructed. First argument is the follow up time second argument is the time of exposure.

- lag
a single positive number giving the time lag between for a concurrent effect to occur (i.e., the TDC at time of exposure

`t-lag`

affects the hazard in the interval containing follow-up time`t`

). Defaults to 0.- formula
A two sided formula with a

`Surv`

object on the left-hand-side and covariate specification on the right-hand-side (RHS). The RHS can be an extended formula, which specifies how TDCs should be transformed using specials`concurrent`

and`cumulative`

. The left hand-side can be in start-stop-notation. This, however, is only used to create left-truncated data and does not support the full functionality.- special
The name of the special whose existence in the

`formula`

should be checked