#{
  Produce a zigzag matrix. Nigel Galloway, January 26th., 2020.
  At the time of writing the Rascal solution is yellow flagged for producing a striped matrix.
  Let me make the same faux pas.
#}
n=5; g=1;
for e=1:n i=1; for l=e:-1:1 zig(i++,l)=g++; endfor endfor
for e=2:n i=e; for l=n:-1:e zig(i++,l)=g++; endfor endfor
#{
  I then have the following, let me call it zig.
    1    2    4    7   11
    3    5    8   12   16
    6    9   13   17   20
   10   14   18   21   23
   15   19   22   24   25

  To avoid being yellow flagged I must convert this striped matrix into a zigzag matrix.
#}
zag=zig'
#{
  So zag is the transpose of zig.
    1    3    6   10   15
    2    5    9   14   19
    4    8   13   18   22
    7   12   17   21   24
   11   16   20   23   25
#}
for e=1:n for g=1:n if(mod(e+g,2))==0 zagM(e,g)=1; endif endfor endfor; zigM=1-zagM;
#{
  I now have 2 masks:

  zigM =

   0   1   0   1   0
   1   0   1   0   1
   0   1   0   1   0
   1   0   1   0   1
   0   1   0   1   0

  zagM =

   1   0   1   0   1
   0   1   0   1   0
   1   0   1   0   1
   0   1   0   1   0
   1   0   1   0   1
#}
zigzag=zag.*zagM+zig.*zigM;
#{
  zigzag =

    1    2    6    7   15
    3    5    8   14   16
    4    9   13   17   22
   10   12   18   21   23
   11   19   20   24   25
#}