ross.moore at mq.edu.au
Fri Aug 24 22:10:04 CEST 2012
On 24/08/2012, at 8:11, <SteveBravy at aol.com> wrote:
> Hello Folks:
> I'm new to XY-pic, and find \POS a little mysterious. I've searched but have not found satisfactory explanations
> 1, What does "interpreting" mean?
> 2. Can I get an explanation of what \POS does?
\POS starts a sequence of reading a character at a time, interpreting what that character means in the current context, then reading extra numerical parameters that may be required, before moving on to the next character. This is also called ‘parsing’ the input stream. It is quite different to using macros or active characters to pass input parameters.
As soon as the parsing sequence encounters something that is not recognised as being appropriate at that place in the parsing stream, then parsing finishes and normal interpretation of the input characters resumes.
If a mistake is made, you can get the subsequent characters showing in the diagram, as if part of a mathematical expression.
Many Xy-pic high-level macros end by calling \POS, e.g, \xy , \xymatrix , \xygraph , \save , \restore , etc.
Hope this helps,
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