SPECTRUM

        SPECTRUM I [J [K]]

    Extract a spectrum from an image according to the following numbering :
        Spectrum (l) = Image (l,i,j,k)
    for a 4-dimensional image. The spectrum is loaded into the  X,Y  buffers
    of  GreG.  It  can  be  processed later by the standard commands LIMITS,
    CONNECT, CURVE and so on.