Get the operator type used in an call, formula, expression, etc.
op.type(x)
# S4 method for class 'call'
op.type(x)
# S4 method for class 'formula'
op.type(x)
# S4 method for class 'ANY'
op.type(x)
# S4 method for class 'expression'
op.type(x)
# S4 method for class 'list'
op.type(x)a character vector of the operator type(s)