Unlike the old expander, the new expander deals with trees instead of
strings. At first, we will create concrete syntax trees which correspond
to the given Macro COSY grammar. Then, we will parse the Macro COSY
program represented as a string into a syntax tree. Finally, the expander
expands the Macro COSY program from its corresponding syntax tree into another
syntax tree which is in the form of a Basic COSY syntax tree, and then,
converts this resulting syntax tree into a string which is in the form of
a Basic COSY program. Now, let's use rule M1 as an example to see how the
old expander and new expander work (For more information, please see Appendix).
The following figure shows how the old expander works:
Productions
specifies
M1. <mprogram> = "program" <mprogrambody> "endprogram ----->
Sets of strings(1) Sets of strings(2)
old expand
MPROGRAM ----------> EBPROGRAM
| |
{"program"} o MPBODY o {"endprogram"} {"program"} o EBBODY o {"end"}
where the string which is inside 
denotes a set of strings, the
operator o denotes string set concatenation,
o : string set * string set -> string set
uppercase names denote sets of strings, for example,
From the above figure we can see that the rule M1 can be specified as sets of
strings(the column of ``Sets of strings(1)''). The old expander can expand these
sets of strings into other set of strings which is the form of basic
COSY program (the column of ``Sets of strings(2)'').
The following figure shows how the new expander works:
Productions
specifies
M1. <mprogram> = "program" <mprogrambody> "endprogram ----->
concrete data structure concrete data structure
or tree for mprogram or tree for ebprogram
new expand
Program ----------> Program
| |
<mprogrambody> <ebprogrambody>
where the non-terminal symbol inside 
denotes a subtree, for instance,
<mprogrambody> denotes the macro program body subtree, etc.,
the prefix m stands for macro, eb stands for expanded basic.
From the above figure we can see that the rule M1 can be specified as a tree (the column of concrete data structure or tree for mprogram). The new expander can expand the tree to another tree which is in the form of a basic COSY program (the column of concrete data structure or tree for ebprogram).
Because of lack of time and in order to test the result, the program we designed expands the Macro COSY program from its corresponding syntax tree into a string directly, and the string is in the form of the Basic COSY program.
In this section, we only discuss the expander. We will discuss the syntax tree and parser later.