code: protected boolean pullLit(final int lit) {
          if (val(lit) == VALUE_TRUE) { return false; }
          if (marked(lit) != 0) { return false; }
          mark(lit);
          bumpLit(lit);
          if (var(lit).level() == this.level) { return true; }
          markFrame(lit);
          this.addedlits.push(lit);
          return false;
      }
docs: Analyzes a given literal.
  @param lit the literal
  @return the result of the analysis