Prevent parent scope variables from leaking into function

locala.ps1

# variable in parent scope
$a = 1

function f1 ($a) { 
    # $a is shadowed by the parameter result is -- 
    "-$a-" 
}

f1 

function f1 { 
    # $a parameter was removed $a comes from
    # parent scope, result is -1-
    "-$a-" 
}

f1

function f1 { 
    # $a parameter was removed so local $a is not
    # defined
    # result is --
    "-$local:a-" 
}

f1