# untangle.specials

From survival v3.1-8
by Terry Therneau

##### Help Process the `specials' Argument of the `terms' Function.

Given a `terms`

structure and a desired special name, this returns an
index appropriate for subscripting the `terms`

structure and another
appropriate for the data frame.

- Keywords
- survival

##### Usage

`untangle.specials(tt, special, order=1)`

##### Arguments

- tt
a

`terms`

object.- special
the name of a special function, presumably used in the terms object.

- order
the order of the desired terms. If set to 2, interactions with the special function will be included.

##### Value

a list with two components:

a vector of variable names, as would be found in the data frame, of the specials.

a numeric vector, suitable for subscripting the terms structure, that indexes the terms in the expanded model formula which involve the special.

##### Examples

```
# NOT RUN {
formula<-Surv(tt,ss)~x+z*strata(id)
tms<-terms(formula,specials="strata")
## the specials attribute
attr(tms,"specials")
## main effects
untangle.specials(tms,"strata")
## and interactions
untangle.specials(tms,"strata",order=1:2)
# }
```

*Documentation reproduced from package survival, version 3.1-8, License: LGPL (>= 2)*

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