1 SUM
        SUM

    This program is used to sum many images,  handling  a  weight  image  in
    order to produce the average image later on.
              It computes     X(i,j) = X(i,j) + F * Z(i,j)
              and             Y(i,j) = Y(i,j) + F
    for  all  non  blanked  pixels  of  the  input  image  Z.   When  called
    repetitively  with  the same input/output images X and Y, X contains the
    weighted sum of all Z images, and  Y  the  weight  of  each  pixels.   A
    parameter is provided to initialise the process.

    The last step to obtain the average image is to  divide  X  by  Y  using
    program COMBINE with option DIVIDE.  You can also use SIC command LET to
    do so, but SUM and COMBINE will handle properly blanking values.
2 START$
        TASK\LOGICAL "Initialize weight and sum files" START$

    Answer YES when you work with the first image, NO afterwards.
2 Z_NAME$
        TASK\FILE "Input image" Z_NAME$

    The name of the file you want to add to the running sum.
2 Y_NAME$
        TASK\FILE "Weight file" Y_NAME$

    The name of the accumulated weight file.
2 X_NAME$
        TASK\FILE "Sum file" X_NAME$

    The name of the accumulated sum file.
2 WEIGHT$
        TASK\REAL "Weight of current image" WEIGHT$

    The relative weight you want to attribute to the current image.
1 END