As far as possible, the mrs DTD is compatible with the rmrs DTD.

There is a slot for an index

cfrom and cto are optional (and not yet supported in the LKB code)

the pred/spred distinction is to allow an MRS to be read in exactly as 
it was.  spred is for predicates which are strings.  this dtd
supports realpred, as in rmrs, but the LKB code doesn't do that yet.

Since we don't have a fixed inventory of sortal values for MRS, this dtd
supports writing the values out much as they are in the `simple' format
via:
<!ELEMENT extrapair (path,value)>
<!ELEMENT path (#PCDATA)>
<!ELEMENT value (#PCDATA)>