parse_mode Function

  character(3) function parse_mode(mode) result(mode_)
    character(*), intent(in) :: mode

    integer :: i
    character(:),allocatable :: a
    logical :: lfirst(3)

    mode_ = 'r t'

    if (len_trim(mode) == 0) return
    a=trim(adjustl(mode))

    lfirst = .true.
    do i=1,len(a)
      if (lfirst(1) &
          .and. (a(i:i) == 'r' .or. a(i:i) == 'w' .or. a(i:i) == 'a' .or. a(i:i) == 'x') &
          ) then
        mode_(1:1) = a(i:i)
        lfirst(1)=.false.
      else if (lfirst(2) .and. a(i:i) == '+') then
        mode_(2:2) = a(i:i)
        lfirst(2)=.false.
      else if (lfirst(3) .and. (a(i:i) == 't' .or. a(i:i) == 'b')) then
        mode_(3:3) = a(i:i)
        lfirst(3)=.false.
      else if (a(i:i) == ' ') then
        cycle
      else if(any(.not.lfirst)) then
        error stop "Wrong mode: "//trim(a)
      else
        error stop "Wrong character: "//a(i:i)
      endif
    end do

  end function parse_mode