The binary operator `:`

has two meanings: for factors `a:b`

is
equivalent to `interaction(a, b)`

(but the levels are
ordered and labelled differently).

For other arguments `from:to`

is equivalent to `seq(from, to)`

,
and generates a sequence from `from`

to `to`

in steps of `1`

or `-1`

. Value `to`

will be included if it differs from
`from`

by an integer up to a numeric fuzz of about `1e-7`

.
Non-numeric arguments are coerced internally (hence without
dispatching methods) to numeric---complex values will have their
imaginary parts discarded with a warning.