src/Composer/DependencyResolver/Solver.php
protected function propagate($level)
    {
        while ($this->decisions->validOffset($this->propagateIndex)) {
            $decision = $this->decisions->atOffset($this->propagateIndex);

            $conflict = $this->watchGraph->propagateLiteral(
                $decision[Decisions::DECISION_LITERAL],
                $level,
                $this->decisions
            );

            $this->propagateIndex++;

            if ($conflict) {
                return $conflict;
            }
        }

        return null;
    }
protected function propagate($level)
    {
        while ($this->decisions->validOffset($this->propagateIndex)) {
            $decision = $this->decisions->atOffset($this->propagateIndex);

            $conflict = $this->watchGraph->propagateLiteral(
                $decision[Decisions::DECISION_LITERAL],
                $level,
                $this->decisions
            );

            $this->propagateIndex++;

            if ($conflict) {
                return $conflict;
            }
        }

        return null;
    }
Makes a decision and propagates it to all rules.

Evaluates each term affected by the decision (linked through watches)
If we find unit rules we make new decisions based on them

@param  int       $level
@return Rule|null A rule on conflict, otherwise null.