2 prompt - display a prompt and wait for input from terminal
13 When prompt(str) is called and input is from a terminal, the string
14 str is displayed and execution is halted until a newline ends a line
15 read from the input; the string formed by the characters in the line
16 (other than the newline) is returned as the value of prompt().
24 > for (;;) {s = prompt("? "); if (s=="end") break; print "\t":eval(s)^2;}