a tiny bit of docs

`rustdoc` won't see it, though, because then `cnf.rs` would have to be a
module.

rust/dpll/cnf.rs

@@ -16,6 +16,22 @@ struct CNF {
    clauses: Vec<Vec<i32>>
}
/// Parses a CNF formula in DIMAC format.
///
/// # Examples
///
/// A simple example for a formula in DIMAC format is below:
///
/// ```text
/// p cnf 5 3
/// 1 2 3 0
/// -2 3 4 0
/// 4 5 0
/// ```
///
/// The above represents a formula with 5 variables and 3 clauses.  The first
/// line specifies this.  Each following line represents a clause with possibly
/// negated literals, terminated by 0 and a newline.
fn parse_dimac(dimac: &str) -> Result<CNF, String> {
    let mut lines = dimac.lines();
    let mut num_vars;