Converts a canonical rational literal of type :real to an OCaml number.
DESCRIPTION
The call rat_of_term t where term t is a canonical rational literal of type
:real returns the corresponding OCaml rational number (type num). The
canonical literals are integer literals &n for numeral n, -- &n for a
nonzero numeral n, or ratios &p / &q or -- &p / &q where p is nonzero,
q > 1 and p and q share no common factor.
FAILURE CONDITIONS
Fails when applied to a term that is not a canonical rational literal.
EXAMPLE
# rat_of_term `-- &22 / &7`;;
val it : num = -22/7