
- Do these belong in vector.tools package?
  - R/which.class
  - test/is-cat.R

- expression.tools package(?)
  - This should probably be called expression.tools rather than formula tool
  - This packages handles calls and expression and assignments (only <- so far).
  - A formula is also a call: 
      is.call( a ~ b )
      [1] TRUE

- implement over custom operators. 
  ? testing for %.+% on the first element
  ? test for name 

- See if rhs, lhs, op, op.type are needed for other assignment operators: <<- ->
