ex = x - 2y

ex = :(x - 2y)

dump(ex)

macro reverse(ex)
    if isa(ex, Expr) && ex.head == :call
        return Expr(:call, ex.args[1], reverse(ex.args[2:end])...)
    else
        return ex
    end
end

# equivalent to 3 - 1
@reverse 1 - 4

macro horner(x, c...)
    ex = esc(c[end])
    for i = length(c)-1:-1:1
        ex = :($(esc(c[i])) + t * $ex)
    end
    return Expr(:block, :(t = $(esc(x))), ex)
end

function my_erfinv(x::Float32) # specialized for single-precision args
    a = abs(x)
    if a >= 1.0f0
        if x == 1.0f0
            return inf(Float32)
        elseif x == -1.0f0
            return -inf(Float32)
        end
        throw(DomainError())
    elseif a <= 0.75f0 # Table 10 in Blair et al.                               
        t = x*x - 0.5625f0
        return x * @horner(t, -0.13095_99674_22f2,
                               0.26785_22576_0f2,
                              -0.92890_57365f1) /
                   @horner(t, -0.12074_94262_97f2,
                               0.30960_61452_9f2,
                              -0.17149_97799_1f2,
                               0.1f1)
    elseif a <= 0.9375f0 # Table 29 in Blair et al.                             
        t = x*x - 0.87890625f0
        return x * @horner(t, -0.12402_56522_1f0,
                               0.10688_05957_4f1,
                              -0.19594_55607_8f1,
                               0.42305_81357f0) /
                   @horner(t, -0.88276_97997f-1,
                               0.89007_43359f0,
                              -0.21757_03119_6f1,
                               0.1f1)
    else # Table 50 in Blair et al.                                             
        t = 1.0f0 / sqrt(-log(1.0f0 - a))
        return @horner(t, 0.15504_70003_116f0,
                          0.13827_19649_631f1,
                          0.69096_93488_87f0,
                         -0.11280_81391_617f1,
                          0.68054_42468_25f0,
                         -0.16444_15679_1f0) /
              (copysign(t, x) *
               @horner(t, 0.15502_48498_22f0,
                          0.13852_28141_995f1,
                          0.1f1))
    end
end
@vectorize_1arg Real my_erfinv

using PyCall
@pyimport scipy.special as s
x = rand(10^7);
@time erfinv(x);
@time s.erfinv(x);
norm(erfinv(x) - s.erfinv(x)) / norm(erfinv(x))