Tilde is used to separate the left- and right-hand sides in a model formula.

`y ~ model`

y, model

symbolic expressions.

The left-hand side is optional, and one-sided formulae are used in some contexts.

A formula has mode `call`

. It can be subsetted by
`[[`

: the components are `~`

, the left-hand side (if
present) and the right-hand side *in that order*.

Chambers, J. M. and Hastie, T. J. (1992)
*Statistical models.*
Chapter 2 of *Statistical Models in S*
eds J. M. Chambers and T. J. Hastie, Wadsworth & Brooks/Cole.