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