Why is my JavaScript function sometimes “not defined”?

function copyArray( pa ) {
    var la = [];
    for (var i=0; i < pa.length; i++)
        la.push( pa[i] );
    return la;
}

Function.prototype.bind = function( po ) {
    var __method = this;
    var __args = [];

    // Sometimes errors -- in practice I inline the function as a workaround.
    __args = copyArray( arguments );

    return function() {
        /* bind logic omitted for brevity */
    }
}

<SCRIPT src="mycode.js"/>

<SCRIPT src="mycode.js"></SCRIPT>

__args = Array.prototype.slice.call(arguments);

(function() {
    function copyArray(pa) {
        // Details
    }

    Function.prototype.bind = function ( po ) {
        __args = copyArray( arguments );
    }
})();