We could introduce ;; as an operator that is like ';', except at top level, no
output values are recorded and the line number is not incremented.  This would
be a convenient way of inserting statements with no effect on subsequence line
numbers, for convenience in comparing the output.

    i1 : 1;;
    stdio:1:3:(1):[0]: error: syntax error at ';'

    i2 : 1;;3
    stdio:1:6:(1):[0]: error: syntax error at ';'
