Inject codes to an extension?

[txtl]

I'm thinking of using PathStyle extension to render the gap with color. I think that if I can inject codes to replace 'discard' with vColor = gapColorInstance, then I can get a color rendered in the dashgap.

I found examples to extend a layer, but I couldn't find an example to extend an extension.

if (gapLength > 0.0 && unitOffset > solidLength) {
      if (capType <= 0.5) {
        if (!(dashGapPickable && picking_uActive)) {
          discard;
        }
      } else {
        // caps are rounded, test the distance to solid ends
        float distToEnd = length(vec2(
          min(unitOffset - solidLength, unitLength - unitOffset),
          vPathPosition.x
        ));
        if (distToEnd > 1.0) {
          if (!(dashGapPickable && picking_uActive)) {
            discard;
          }
        }
      }
    }
  }

[txtl]

Found an example in the document.