The Access to the Metarules through the XTAG Interface

We first describe the access to the metarules subsystem using buffers with single metarule applications. Then we proceed by describing the application of multiple metarules in what we call the parallel, sequential, and cumulative modes to input tree files. We have defined conceptually a metarule as an ordered pair of trees. In the implementation of the metarule subsystem it works the same: a metarule is a buffer with two trees. The name of the metarule is the name of the buffer. The first tree that appear in the main window under the metarule buffer is the left hand side, the next appearing below is the right hand side^25.12. The positional approach allows us to have naming freedom: the tree names are irrelevant^25.13. Since we can save buffers into text files, we can talk also about metarule files. The available options for applying a metarule which is in a buffer are:

• For applying it to a single input tree, click in the name of the tree in the main window, and choose the option apply metarule to tree. You will be prompted for the name of the metarule to apply to the tree which should be, as we mentioned before, the name of the buffer that contains the metarule trees. The output trees will be generated at the end of the buffer that contains the input tree. The names of the trees depend of a LISP parameter *metarules-change-name* . If the value of the parameter is false -- the default value -- then the new trees will have the same name as the input, otherwise, the name of the input tree followed by a dash (`-') and the name of the right hand side of the tree^25.14. The value of the parameter can be changed by choosing Tools at the menu bar and then either name mr output trees = input or append rhs name to mr output trees.
• For applying it to all the trees of a buffer, click in the name of the buffer that contains the trees and proceed as above. The output will be a new buffer with all the output trees. The name of the new buffer will be the same as the input buffer prefixed by "MR-". The names of the trees follow the conventions above.

The other options concern application to files (instead of buffers). Lets first define the concepts of parallel, sequential and cumulative application of metarules. One metarule file can contain more than one metarule. The first two trees, i.e., the first tree pair, form one metarule - lets call it mr₀. Subsequent pairs in the sequence of trees define additional metarules -- mr₁, mr₂, ..., mr_n.

• We say that a metarule file is applied in parallel to a tree (see Figure B.5) if each of the metarules is applied independently to the input generating its particular output trees^25.15. We generalize the concept to the application in parallel of a metarule file to a tree file (with possibly more than one tree), generating all the trees as if each metarule in the metarule file was applied to each tree in the input file.
  
    

  Figure B.5: Parallel application of metarules

  [scale=0.7]/mnt/linc/xtag/work/doc/tech-rept/fig/parallel.ps
  
  
• We say that a metarule file mr₀, mr₁, mr₂, ...,mr_n is applied in sequence to a input tree file (see Figure B.6) if we apply mr₀ to the trees of the input file, and for each apply metarule mr_i to the trees generated as a result of the application of mr_i-1.
  
    

  Figure B.6: Sequential application of metarules

  [scale=0.7]/mnt/linc/xtag/work/doc/tech-rept/fig/sequential.ps
  
  
• Finally, the cumulative application is similar to the sequential, except that the input trees at each stage are by-passed to the output together with the newly generated ones (see Figure B.7).
  
    

  Figure B.7: Cumulative application of metarules

  [scale=0.7]/mnt/linc/xtag/work/doc/tech-rept/fig/cumulative.ps
  
  

Remember that in case of matching failure the output result is decided as explained in subsection  B.2.3 either to be empty or to be the input tree. The reflex here of having the parameter set for copying the input is that for the parallel application the output will have as many copies of the input as matching failures. For the sequential case the decision apply at each level, and setting the parameter for copying, in a certain sense, guarantees for the 'pipe' not to break. Due to its nature and unlike the two other modes, the cumulative application is not affected by this parameter. The options for application of metarules to files are available by clicking at the menu item Tools and then choosing the appropriate function among:

• Apply metarule to files: You'll be prompted for the metarule file name which should contain one metarule^25.16, and for input file names. Each input file name inpfile will be independently submitted to the application of the metarule generating an output file with the name MR-inpfile.
• Apply metarules in parallel to files: You'll be prompted for the metarules file name with one or more metarules and for input file names. Each input file name inpfile will be independently submitted to the application of the metarules in parallel. For each parallel application to a file inpfile an output file with the name MRP-inpfile will be generated.
• Apply metarules in sequence to files: The interaction is as described for the application in parallel, except that the application of the metarules are in sequence and that the output files are prefixed by MRS- instead of MRP-.
• Apply metarules cumulatively to files: The interaction is as described for the applications in parallel and in sequence, except that the mode of application is cumulative and that the output files are prefixed by MRC-.

Finally still under the Tools menu we can change the setting of the parameter that controls the output result on matching failure (see Subsection B.2.3) by choosing either copy input on mr matching failure or no output on mr matching failure.