convert back to OpenMath dummy yacasform

[MarcoCostantini]

This is the opposite of #300 Let's consider an OpenMath object unknown to Yacas:

<OMOBJ xmlns="http://www.openmath.org/OpenMath" version="2.0"><OMS cd="foo" name="bar"/></OMOBJ> 

if this object is converted to Yacas, then a dummy object yacasform is created, and this is OK:

In> FromString("<OMOBJ xmlns=\"http://www.openmath.org/OpenMath\" version=\"2.0\"><OMS cd=\"foo\" name=\"bar\"/></OMOBJ> ")OMRead()
Out> yacasform

Of course, it's not possible to do much with this dummy object, but it should be possible at least to convert it back to OpenMath, to get the starting object, but it doesn't work:

In> OMForm(%)
<OMOBJ>
  <OMV name="yacasform"/>
</OMOBJ>
Out> True

The same for OMA OpenMath objects:

In> FromString("<OMOBJ xmlns=\"http://www.openmath.org/OpenMath\" version=\"2.0\"><OMA><OMS cd=\"foo\" name=\"bar\"/><OMI>1</OMI><OMI>2</OMI><OMI>3</OMI></OMA></OMOBJ> ")OMRead()
Out> yacasform(1,2,3)
In> OMForm(%)
<OMOBJ>
  <OMA>
    <OMS cd="yacas" name="yacasform"/>
    <OMI>1</OMI>
    <OMI>2</OMI>
    <OMI>3</OMI>
  </OMA>
</OMOBJ>
Out> True
In>