 C++ expression. 
	As an example, the expression
	<code class="literal">_1 + _2</code> creates a binary lambda functor. 
	It can be called with two objects of any types <code class="literal">A</code> and <code class="literal">B</code> for which <code class="literal">operator+(A,B)</code> is defined (and for which BLL knows the return type of the operator, see below).
      </p>
<p>
	C++ lacks a mechanism to query a type of an expression. 
	However, this precise mechanism is crucial for the implementation of