An Atom.T is a unique representative for a set of equal texts (like a Lisp atomic symbol) \index{atomic symbol} \index{symbolic expression!atom}

INTERFACE Atom;

TYPE T <: REFANY;

CONST Brand = "Atom-1.0";

PROCEDURE FromText(t: TEXT): T;

Return the unique atom a such that for any text u, if Text.Equal(u, t), then FromText(u) = a.

PROCEDURE ToText(a: T): TEXT;

Return a text t such that FromText(t) = a.

PROCEDURE Equal(a1, a2: T): BOOLEAN;

Return a1 = a2.

PROCEDURE Hash(a: T): INTEGER;

Return a hash code for a by taking the image of ToText(a) under some fixed hash function.

PROCEDURE Compare(a1, a2: T): [-1..1];

Cause a checked runtime error.

END Atom.