sourcewalk (etc) drops all but the first character in replacements

[oxinabox]

This is hard to explain.
here is a demo

I want to remove the second argument of a 3 argument function.

julia> using MacroTools: textwalk

julia> code = """
           test_scalar(cos, x -> -sin(x), x)
           test_scalar(tan, x -> 1 + tan(x)^2, x)
           test_scalar(sec, x -> sec(x) * tan(x), x)
           test_scalar(csc, x -> -csc(x) * cot(x), x)
           """;

julia> function twoarg(expr)
       @capture(expr, t_(f_, d_, x_)) && t==:test_scalar && return :(test_scalar($(f), $x))
       return expr
       end
twoarg (generic function with 1 method)

julia> textwalk(twoarg, code) |> println
test_scalar(c, x)
test_scalar(t, x)
test_scalar(s, x)
test_scalar(c, x)