Property merge & last-occurrence anchoring

Jess supports Less-style property merge for aggregating several declarations of the same property into one comma- or space-separated value:

• +: — comma merge (values join with , )
• +_: — space merge (values join with a single space)

.mixin() {
  box-shadow+: inset 0 0 10px #555;
}
.myclass {
  .mixin();
  box-shadow+: 0 0 20px black;
}

.myclass {
  box-shadow: inset 0 0 10px #555, 0 0 20px black;
}

Last-occurrence anchoring

When a property is merged across several declarations, the combined value renders at the position of the last occurrence in source order. (Less 4.x anchored at the first occurrence; Jess and the Less 5.x engine anchor at the last — a deliberate divergence.)

It's a self-reference under the hood

A merge declaration lowers to a plain self-referencing declaration: the property reads its own prior value and appends to it. You can write the self-reference explicitly with the property-access form instead of the + sugar:

.myclass {
  box-shadow: inset 0 0 10px #555;
  box-shadow: $['box-shadow'], 0 0 20px black;  // same result
}

If there is no prior value, the self-reference is empty and the leading separator is dropped, so the first occurrence prints cleanly. !important on any member of the chain propagates to the merged output.

See also the Less Merge Operators page.