Check that another offset is not specified

[dill]

It's currently totally legal to write a dsm formula with an offset in it. This throws the following warning:

Warning message:
In av[kp] <- as.character(attr(tf, "variables")[1 + off]) :
  number of items to replace is not a multiple of replacement length

Usually the user will not want 2 offsets, so we should stop them doing this.

[dill]

This can be checked using the terms function and looking at the resulting attributes:

> terms(formula(x~y + offset(a)))
x ~ y + offset(a)
attr(,"variables")
list(x, y, offset(a))
attr(,"offset")
[1] 3
attr(,"factors")
          y
x         0
y         1
offset(a) 0
attr(,"term.labels")
[1] "y"
attr(,"order")
[1] 1
attr(,"intercept")
[1] 1
attr(,"response")
[1] 1
attr(,".Environment")
<environment: R_GlobalEnv>

so here attr(terms(formula(x~y + offset(a))), "offset") will give the position of the offset term. Whereas when there is no offset:

> terms(formula(x~y))
x ~ y
attr(,"variables")
list(x, y)
attr(,"factors")
  y
x 0
y 1
attr(,"term.labels")
[1] "y"
attr(,"order")
[1] 1
attr(,"intercept")
[1] 1
attr(,"response")
[1] 1
attr(,".Environment")
<environment: R_GlobalEnv>

There is no offset attribute. So testing for presence of that attribute should be enough.