All these XObjects (PDF External Objects) are intended to be included in a
PDF file.

Their object number are all the same (0), but the PDF driver will attribute
to them a unique number during the insertion of these objects in the PDF
file.

All these objects must define a stream length (field /Length), eventually
equal to zero (the actual size will be computed during the closing of the
PDF document). The total number of characters (or spaces) after the /Length
keyword and before the line termination must be at least equal to 10.

If you add comments in these PDF fragments, they cannot be located between
'stream' and 'endstream'.

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If you wish to change the shape of a symbol (even just for try), you could
change the PDF commands in the file "The 26 marker symbols.pdf" (using any
text editor), then render the PDF file using your favorite visualizer.

If you modify actually an XObj file, it will be used (under the condition
of a valid modification) in the printed PDF, but not in the printed EPS nor
in the X11 figure on the screen. A similar modification could be made in the
Fortran source code for the EPS generation, but for the X11 rendering,
another modified version of the font file CeMaSP.pfb is needed.

Creating additional marker symbols is even more difficult, because they need
to be all registered in the Fortran source code.

In future, I may implement a mechanism to easily accept user-defined marker
symbols, but anyway this requires to understand both the PS and PDF language.
Not sure that this can be built without a recompilation of the whole Muesli
library by the user.

