interp 0
macro interp [1]:
        irps i, expr
Processed: irps i, 0
Error: illegal instruction.    

macro run? expr&
    local b, i, x
    match x, expr
        display `x
        irps b, x
            display 'a'
        end irps
    end match
end macro
run k+1

    match x, k+1
        display `x
        irps b, x
            display 'b'
        end irps
    end match