3 Syntax
Atoms start with lower-case letters and can include letters, numbers
and underscore ("_")
A body is either an atom, of the form
alpha&beta, where alpha and beta are bodies,
or of the form ~alpha, where alpha is a body.
A clause is either an atom or is a rule of the form
h<-b where h is an atom (the head of the clause) and b
is a body.
