Hi all,

I am looking at what sort of grammar can be used to represent and edit 
the content MathML included inside components and reactions in CellML 
models. I propose the following grammar (this is a valid and unambiguous 
bison grammar). This has been designed to be intuitive to people who 
know MATLAB (or C, or other languages which use a mixture of pre and 
in-order operators). It would be nice if there was some consistency 
between tools, so I am interested in feedback both from potential users, 
and the developers of other tools.

%left '!'
%left '+' '-'
%left '*' '/'

textrep: expr '=' attrs expr | textrep '=' attrs expr;
attrs: '{' attrlist '}' | /* empty */;
attrlist: attr attrlist | /* empty */;
arglist: expr ',' arglist | expr;
expr: IDENTIFIER attrs '(' arglist ')' | '(' expr ')' | expr '*' attrs 
expr |
  expr '+' attrs expr | expr '-' attrs expr | expr T_EQEQ attrs expr |
  expr T_NOTEQ attrs expr | expr '<' attrs expr |
  expr '>' attrs expr | expr T_GREATEREQUAL attrs expr |
  expr T_LESSEQUAL attrs expr | expr '/' attrs expr | IDENTIFIER attrs |
  NUMBER attrs | PIECEWISE attrs '(' caselist maybeelse ')' | '!' attrs 
expr |
  '-' attrs expr;
caselist: casepair caselist | /* empty */;
casepair: CASE expr THEN expr;
maybeelse: ELSE expr | /* empty */;

Examples of valid syntax:
x ={id="myequation"} piecewise(case y==1 then abs(z*y) / 
integral(sin(a), a, 0, 10) case y < 1 then pow(y, z) else -(y + z - 
y * z = a * b = c / pow(2, remainder(5, 3)) / 7{units="second"}
diff(position, time) = velocity
a = 
* x)

There is one minor issue with the identifier syntax, and that is the 
possibility that names can conflict, e.g. if we had a variable called 
pi. We would have to mangle it (I suggest that the tokeniser would use # 
to override symbol names, so, for example #pi maps to <ci>pi</ci>, while 
pi maps to <pi/>, but of course, # is not normally required, so x maps 
to <ci>x</ci>).

My editing syntax allows the encoding of things which are valid MathML, 
but which no tools will be able to be used (e.g a CellML tool would be 
able to integrate equations like the last one, as the semantics of sine 
have been overriden using definitionURL). The aim is to represent all 
valid content MathML, rather than everything that PCEnv or any other 
tool supports. This means that going in to edit mode on an equation, not 
changing anything, and then returning back is guaranteed not to change 
the semantic meaning of the model.

The units attribute will default to dimensionless if not present, but 
can be explicitly set on constants.

For the IDENTIFIER attrs '(' arglist ')', all standard MathML operators 
(other than those for which an in-order operator is defined) will be 
supported as IDENTIFIER, as well as some special values for tagging 
certain qualifiers (the integral example above uses lowlimit and 
uplimit, which are inferred from their position, but you could 
alternatively write something like
integral(sin(a), a, condition(in(a, interval{type="open"}(0, 10))) ).


Best regards

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