  We prove L^p estimates for a tri-linear operator, whose symbol is given by
the product of two standard symbols, satisfying the well known
Marcinkiewicz-Hormander-Mihlin condition. Our main result contains in
particular the classical Coifman-Meyer theorem. This tri-linear operator is the
simplest example of a large class of multi-linear operators, which we called
"paraproducts with flag singularities".
