1. julia> "bar" foo()=nothing
    2. foo
    4. help?> foo
    5. search: foo floor pointer_from_objref OverflowError RoundFromZero unsafe_copyto! functionloc StackOverflowError
    7.   bar

    在输入后按2次Tab

    1. help?> is
    3. isa             isconcretetype   isfifo           islowercase      ispath           issetequal       istaskfailed
    4. isabspath       isconst          isfile           ismarked         isperm           issetgid         istaskstarted
    5. isabstracttype  isdefined        isfinite         ismissing        ispow2           issetuid         istextmime
    6. isapprox        isdigit          isimmutable      ismount          isprimitivetype  issocket         isuppercase
    7. isascii         isdir            isinf            ismutable        isprint          issorted         isvalid
    8. isassigned      isdirpath        isinteger        isnan            ispunct          isspace          iswritable
    9. isbits          isdisjoint       isinteractive    isnothing        isqrt            issticky         isxdigit
    10. isbitstype      isdispatchtuple  isless           isnumeric        isreadable       isstructtype     iszero
    11. isblockdev      isempty          isletter         isodd            isreadonly       issubnormal
    12. ischardev       isequal          islink           isone            isready          issubset
    13. iscntrl         iseven           islocked         isopen           isreal           istaskdone
    14. julia> ismutable(String)
    15. true