<Chapter Label="ExtendingOpenMath">
<Heading>Extending the &OpenMath; package</Heading>    

<Section Label="Available">
<Heading>Exploring the range of supported symbols</Heading>
<Index Key="OMsymRecord"><C>OMsymRecord</C></Index>

The &OpenMath; package supports such basic &OpenMath; objects
as integers ( &lt;OMI> ), character strings ( &lt;OMSTR> ) and
variables ( &lt;OMVAR> ). Besides that, it supports a number
of &OpenMath; content dictionaries (some of them only partially,
dependently on their relevance to &GAP;). 

To see which symbols from which content dictionaries are supported
for the conversion from &OpenMath; to &GAP;, explore the global record
<C>OMsymRecord</C>. Its components have names of appropriate CDs,
and subcomponents of each component have names of symbols from the
corresponding CD. If the value of the component is not equal to
<K>fail</K>, then it contains the function or the object which is used 
for conversion. The following example of the entry for the <C>nums1</C> CD
demonstrates a combination of all possible cases:

<Example>
<![CDATA[
gap> Display( OMsymRecord.nums1 );          
rec(
  NaN := nan,
  based_integer := fail,
  e := 2.718281828459045,
  gamma := fail,
  i := E(4),
  infinity := infinity,
  pi := 3.141592653589793,
  rational := function ( x )
        return OMgapId( [ OMgap2ARGS( x ), x[1] / x[2] ] )[2];
    end )
]]>
</Example>

<C>OMsymRecord</C> contains all symbols for which conversion from 
&OpenMath; to &GAP; is supported except some special symbols related
with errors and special procedures from the &SCSCP; package which
are treated separately. 
<P/>

To check quickly if &GAP; can parse a given &OpenMath; object,
copy the &OpenMath; code and paste it directly into standard input
after the following command:

<Log>
<![CDATA[
gap> s:= InputTextUser();; g := OMGetObject(s); CloseStream(s);
]]>
</Log>

The main tool for the conversion from &GAP; to &OpenMath; is 
<C>OMPut( &lt;writer>, &lt;object> )</C>. A number of methods for
<C>OMPut</C> are installed in the file <File>openmath/gap/omput.gi</File>.
<P/>

To check quickly whether the object may be converted to &OpenMath;,
call <C>OMprint</C> for that object, for example:

<Example>
<![CDATA[
gap> OMPrint( [ [1..10], ZmodnZObj(2,6), (1,2) ] );                
<OMOBJ>
	<OMA>
		<OMS cd="list1" name="list"/>
		<OMA>
			<OMS cd="interval1" name="integer_interval"/>
			<OMI>1</OMI>
			<OMI>10</OMI>
		</OMA>
		<OMA>
			<OMS cd="integer2" name="class"/>
			<OMI>2</OMI>
			<OMI>6</OMI>
		</OMA>
		<OMA>
			<OMS cd="permut1" name="permutation"/>
			<OMI>2</OMI>
			<OMI>1</OMI>
		</OMA>
	</OMA>
</OMOBJ>
]]>
</Example>

The package is in the continuous development and will support even more symbols in the future.
In the meantime, if you will have any requests for the support for particular symbols, please
contact Alexander Konovalov.

</Section>

<Section Label="Private">
<Heading>Adding support for private content dictionaries and symbols</Heading>

There is also a way for the user to extend the package adding support for private
and experimental CDs or separate symbols. We allocated the directory <File>openmath/private</File>
for this purposes, and currently it contain the file <File>private.g</File> for conversions
from &OpenMath; to &GAP; and the file <File>private.gi</File> for conversions from &GAP;
to &OpenMath; for some symbols from private CDs contained in the <File>openmath/cds</File> directory.
<P/>

In particular, we extended the package with the following private &OpenMath; symbols:
<List>
<Item>
<C>group1.group_by_generators</C> which allows us to input and output groups given by 
their generators as this is a natural way to create groups in &GAP;;
</Item>
<Item>
<C>semigroup1.semigroup_by_generators</C> and <C>monoid1.monoid_by_generators</C>
following the same considerations for semigroups and monoids;
</Item>
<Item>
<C>pcgroup1.pcgroup_by_pcgscode</C> for PcGroups given by their pcgs code and order;
</Item>
<Item>
<C>record1.record</C> for records as they are important data structures which we want
to pass in a straightforward manner between different &GAP; instances;
</Item>
<Item>
<C>transform1.transformation</C> to support transformations, transformation semigroups 
and their automorphism groups.
</Item>
</List>

The file <File>private.g</File> is loaded from <File>openmath/gap/new.g</File>, 
and the <File>private.gi</File> is loaded from <File>openmath/gap/read.g</File>.
If the user would like to add own code, this may be done either by adding it
to these files or by placing additional files in <File>openmath/private</File>
directory and load them similarly to <File>private.g</File> and <File>private.gi</File>.

We will welcome user's contributions in the form of the code to support existing content
dictionaries from the &OpenMath; web site or private content dictionaries, if they may
be interesting for a wider community.

</Section>

</Chapter>