La and Eu abundances of M 22 as functions of [Fe/H]. In panels a) and b) we repeat the [Eu/Fe] and [La/Fe] panels of Fig. 6. In panel b) we have added a dashed line at [La/Fe] =  + 0.2 to show the division in this abundance ratio between s-rich and s-poor stars; see text for discussion of this choice. In panel c) we plot the [La/Eu] values; the separation between the two groups of stars is more obvious here, and the dashed line represents our chosen split at [La/Eu] =  −0.3. Symbols are as in Fig. 6.