Grice on "E" and "E!"

From M. R.'s entry, nonexistent objects, in online Stanford encyclopedia:

"it has been shown that the distinction between the two can be coherently regimented in various ways. In the systems of Terence Parsons, Edward N. Zalta, and Dale Jacquette, for instance, “there is an x such that … x…” is expressed by

“∃x(…x…)”,

whereas “there exists an x such that … x…” is expressed by

“∃x(E!x & …x…)”,

where “E!” is the existence predicate (Parsons 1980, Zalta 1983, Zalta 1988, Jacquette 1996). “Some things do not exist” could thus be rendered in logical notation as follows: “∃x(¬E!x)”;

“Pegasus does not exist” as

“¬E!p”; and so forth."