Overflows in expressions
========================

    uint i = ...
    uint j = ...
    # here, the operation is "byte - byte",
    # which means that it will trap if:
    #   i>255,
    #   j>255, or
    #   i-j<0
    # The former two cases might not be expected by {C, C++, Java, C#} programmers!
    byte b = i - j