// It's possible to use $reduce to handle the "insert a document
// immediately after (or before) a matching document" use case:
db.marching_order.updateOne( { _id : 'column-one' }, [
  { $set : { marchers : { $reduce : {
    input         : '$marchers',
    initialValue  : [],
    in            : {
      // iterate through the marchers list (an array of documents with
      // a name property), and apply the rules in this $switch to each
      // matching person that we find...
      $switch : {
        // Within the `$reduce`, the `$$this` variable refers to the
        // current element of the list, and the `$$value` variable is
        // the accumulator..  For JavaScript devs it's something like:
        // $marchers.reduce( ( $$value, $$this ) => {} );
        branches  : [
          {
            // When we find "Bob"...
            case : { $eq : [ "$$this.name", "Bob" ] },
            // we want to add "Alice" right before him
            then : { $concatArrays : [
              // Note that the accumulator is already an array
              // (because of the `initialValue : []` up at the
              // beginning)...
              "$$value",
              // But for other values we need to wrap them in an
              // array.
              [ { name : "Alice", age : 11 } ],
              [ "$$this" ],
            ] },
          },
          {
            // When we find "Chuck"...
            case : { $eq : [ "$$this.name", "Chuck" ] },
            // we're going to add 4 people behind him
            then : { $concatArrays : [ "$$value", [
              "$$this",
              { name : "Dave", age : 20 },
              { name : "Dave Too", age : 19 },
              { name : "Also Dave", age : 17 },
              { name : "Elwood", age : 43 },
            ] ] },
          },
          {
            // We're also going to remove everyone from the list who
            // is bad at marching
            case : { $or : [
              // Zeke is terrible at marching
              { $eq : [ "$$this.name", "Zeke" ] },
              // Everyone under 3 just gets in the way
              { $lt : [ "$$this.age", 3 ] },
              // Everyone over 100 ends up needing a wheelchair
              { $gt : [ "$$this.age", 100 ] },
            ] },
            // Returning the accumulator without adding "$$this" to it
            // effectively removes that item from the list.
            then : "$$value",
          },
        ],
        // The default if none of the branches matched is to just add
        // that item the accumulator.
        default : { $concatArrays : [ "$$value", [ "$$this" ] ] },
      },
    },
  } } } }
] );