A vector or a list of grouping elements, each as long as the
variables in x. The elements are coerced to factors before use
(in the sense that interaction(by) defines the grouping).
## Data from Aitchison 1986data("slides")
## Coerce to a compositional matrixcoda <- as_composition(slides, groups = 2)
## Compositional mean by groupcondense(coda)