  This article proposes an initiation to \'Ecalle's mould calculus, a powerful
combinatorial tool which yields surprisingly explicit formulas for the
normalising series attached to an analytic germ of singular vector field. This
is illustrated on the case of saddle-node singularities, generated by
two-dimensional vector fields which are formally conjugate to Euler's vector
field $x^2\frac{\pa}{\pa x}+(x+y)\frac{\pa}{\pa y}$, and for which the formal
normalisation proves to be resurgent in $1/x$.
