Hello, my question: is there a reason why \toksapp assignment cannot be set expandable by \immediateassigned or \immediateassignment? We can compare: \immediateassigned{\toks0={abc} \toks0\ea{\the\toks0 xyz}} it works. On the other hand: \immediateassigned{\toks0={abc} \toksapp0{xyz}} fails with error: ! This can't happen (prefix). l.2 \immediateassigned{\toks0={abc} \toksapp 0{xyz}} Where is a difference? It should be great if \toksapp can be set expandable. I use it in my macro \replstring. If very large strings are modified by this macro at many points then there is significantly better calculation speed with \toksapp than \toks\ea{\the\toks} (approximately ten times better). So \toksapp is optimal but if it is used in a macro then the macro cannot be set expandable. Best regards Petr Olsak