A pattern has emerged of wrapping Expression instances with other
Expressions in order to subtly modify their behavior. A key example of
this is in ext/dynblock, where wrap an expression in order to introduce
our additional iteration variable for expressions in dynamic blocks.
Rather than having each wrapper expression implement wrapping
implementations for our various syntax-level-analysis functions (like
ExprList and AbsTraversalForExpr), instead we define a standard mechanism
to unwrap expressions back to the lowest-level object -- usually an AST
node -- and then use this in all of our analyses that look at the
expression's structure rather than its value.
This extension allows an application to support dynamic generation of
child blocks based on expressions in certain contexts. This is done using
a new block type called "dynamic", which contains an iteration value
(which must be a collection) and a specification of how to construct a
child block for each element of that collection.
