avoiding syntax that is only available with S-Lang >= pre2.2.3-39

change #39 since S-Lang 2.2.2:

Added a syntax enhancement to structs and qualifiers to allow the
fields of structure-valued expressions to be used to define the
structure.  For example:

   space = struct {x, y, z};
   spacetime = struct {@space, t};

Here spacetime is a structure containing 4 fields: (x,y,z,t).  In
this context, @ is not a dereference operator.  Rather it is used to
tell the parser that the following expression is a structure valued.