new problem frown

STRING* a_str = "text one";

TEXT* a_txt =
{//------------------------------------
pos_x = 180;
pos_y = 223;
layer = 4;
font = georgia_font;
string (a_str);
flags = VISIBLE;
}

PANEL* game_field_pan =
{//------------------------------------
pos_x = 509;
pos_y =223;
layer = 3;
bmap = "optionen.bmp";
button (10, 47, "on.bmp", "off.bmp", "over.bmp", NewText, NULL, NULL);
flags = /*OVERLAY*/ | VISIBLE;
}

function NewText()
{//------------------------------------
a_str ="text two";
}


i want to change with a button an existing text, but that script dont work smirk