2.28                      *******************
                                  * MRGMSK WRITE-UP *
                                  *******************
        
           This program is used to merge two different mask files into a 
        single mask file. It is needed when one wants to create an averaging 
        mask from a set of input atomic coordinates, and the 
        noncrystallographic symmetry present is not purely rotational with 
        order of rotation N where N is an integer, i.e. if arbitrary rotations 
        and/or post rotation translations are required. In that case MDLMSK 
        should be run two or more times, with each run generating a mask for a 
        different molecule in the asymmetric unit, and SPECIFYING A UNIQUE 
        MASK NUMBER. Program MRGMSK can then combine all of the individual 
        mask files into one, which can be used for averaging, operator 
        refinement etc. All of the input masks must cover precisely the same 
        map volume, and use the same map grid spacing. The output mask will 
        correspond to this volume and spacing as well. If both of the input 
        masks identify the same grid point as being within its envelope, the 
        status of that point is changed to indicate non-molecule, since it 
        is not clear to which molecule it should belong. The number of such 
        overlapping points is output. The output mask can be examined in 
        MAPVIEW, in which case the different molecular masks will be shown in 
        different colors. This program is interactive and prompts for the 
        input and output mask files, and the standard parameter file.
        
        
                                ******** FILES ********
        
           INPUT (AND OUTPUT) MASK FILES (BINARY)  
        
           record 1) A,B,C,AL,BE,GA,NX,NY,NZ,IXMN,IYMN,IZMN,IXMX,IYMX,IZMX
        
                 with first 6 values REAL*4, next 9 INTEGER*4, lengths in 
                 Angstroms, angles in degrees.
        
                 NX = 
                       Number of grid points defining one "cell length" along
                 NY =  respective axis. Implicitly defines grid spacing as 
                       del x = A/NX, del y = B/NY and del z = C/NZ
                 NZ =
        
         IXMN, IXMX =  
                      Minimum, maximum grid index defining map region such
         IYMN, IYMX = that  x (fractional) = IX * (del x) / A  etc. 
                      There are no restrictions on magnitudes or signs.
         IZMN, IZMX = 
        
        The mask follows as (IYMX-IYMN+1)*(IZMX-IZMN+1) records, with each 
        containing one row (IXMX-IXMN+1 BYTE values) along X, starting at 
        IXMN. Y is slowest varying, i.e. the file could have been created with 
        the following FORTRAN code, where array MASK is defined to be FORTRAN 
        type BYTE:
        
               DO 30 IY=IYMN,IYMX
                       DO 20 IZ=IZMN,IZMX
        20     WRITE(LU)(MASK(IX,IY,IZ),IX=IXMN,IXMX)
        30     CONTINUE