The Seed7 programming language

Lisp has user-defined statements and operators which integrate perfectly into Lisp (look and feel built-in; aren't "second class" in any way). Invocations of these custom forms look like Lisp, just like user-defined Seed7 forms look like ... Seed7!

Infix can be supported as cleanly as you want it; it can be a hack, or not, and can certainly have user-defined operators with custom associativity or precedence.

This can be done without recompiling the underlying Lisp system or touching any of it.

Can we take, out of Seed-7, the ability to define user-defined operators, and put that capability back in the same or very similar form only by writing Seed-7 application code?