MessagePort commands with parameters that get a body as a value

The following is valid AdoScript code, but the CHECKBOX is marked as being wrong:

CC "AdoScript" MLISTBOX entries:(tokstr(arrDbFiles, "\n")) toksep:("\n") title:("Select database files") extra:{
  CHECKBOX "Delete files" checked:0 result-var:deletefiles
}

The AdoScript (and possibly LEO) syntax should be extended to allow for these kinds of constructs where necesssary.