architecture = "architecture rtl of adder is begin" LF "  s <= " expr ";" LF "  co <= " expr ";" LF "end rtl;" LF

expr = var / ( "( " subexpr " )" )

subexpr = ( expr op expr ) / ( "not " expr )

op = " and " / " or " / " nand " / " nor " / " xor " / " xnor "

var = "i0" / "i1" / "ci"

