That microsolver is used in the old SPH fluids to maintain a list of neighbor particles within a search radius. These days you're better off just using pcfind and storing the results in array attributes.
So using a POP or Geometry Wrangle:
// next two variables could be parameters
int maxn = 100;
float scale = 2.5;
float searchrad = f@pscale * scale;
// find nearby particles
int n[] = pcfind(0, "P", @P, searchrad, maxn + 1);
// remove this particle from list
removevalue(neighbors, @ptnum);
i[]@neighbors = n;
On the Inputs tab set Input 1 to Myself or Myself (no reads of Outputs), the latter being faster.
You can then loop through that array to do various neighbor-lookup type things. The POP Grains solver does lots of this if you want to look inside.