An example of how you can detour a user-defined function (UDF) in Expression 2.

function-detour.txt

@persist Function:table
if (first())
{
    Function = table()
    function call(ID)
    {
        timer("function_" + ID, first() ? 0 : 1000)
    }
    call(1)
}
else
{
    local C = clkName()
    local D = C:match("^function_([0-9]+)$")
    if (D:count())
    {
        local I = D[1, string]:toNumber()
        local Signature = ""
        switch (I)
        {
            case 1,
                Signature = format("void=%s()", C)
                function function_1()
                {
                    print("The first function.")
                }
                print(Signature)
                break
            case 2,
                Signature = format("number=%s()", C)
                function function_1()
                {
                    print("Detoured the first function.")
                    #return 2
                }
                function function_2() { function_1() }
                print(Signature)
                break
            default,
                print("Done.")
                stopAllTimers()
                timer(toString(I - 1), 1000)
                exit()
                break
        }
        Function[I, string] = C
        Function[C, string] = Signature
        call(I + 1)
    }
    else
    {
        local Detour = Function[C:toNumber(), string]
        print("Calling detour: " + Detour)
        Detour()
    }
}

simple-function-detour.txt

@persist UserFunction:string
UserFunction = "userFunction"

# Our user-defined function (UDF):
function string userFunction()
{
    return "Hello World"
}
print(UserFunction()[string]) # Hello World

# Detouring the "userFunction" UDF (think of it as a new "userFunction" UDF):
function string userFunction()
{
    # Do something evil...
    local S = "!"
    local I = 1
    while (perf(100)) { S += S:repeat(I) I++ }
    return "Detoured userFunction!"
}
print(UserFunction()[string]) # Detoured userFunction!