On Tue, 20 May 2014, Chris Rowley wrote:

    [...]

    But we may need to distinguish operators from variables.  The variable case would include the case of 'variable names' that include spaces (I find it difficult to imagine an operator name with spaces but who knows?)

counter-examples from the texbook:
\liminf (lim\,inf) and \limsup (lim\,sup).

    [...]

    PS: the term 'identifier' is not, at least not until quite recently, ever used by math folk.

not until some of us had to get into
the font business.  sorry 'bout that.
						-- bb