Swap the allowed nesting of sum and diff expressions: now a diff expression can
contain a sum expression, but only on the left hand side. A difference of two
expressions sum must be canonicalized by grouping their constant terms in a
single expression. This change of sturcture became possible thanks to the
introduction of the "direct" super-kind. It is necessary to enable support of
sum expressions on the left hand side of the stripe expression.
SDBM expressions are now grouped into the following structure
- expression
- varying
- direct
- sum <- (term, constant)
- term
- symbol
- dimension
- stripe <- (term, constant)
- negation <- (direct)
- difference <- (direct, term)
- constant
The notation <- (...) denotes the types of subexpressions a compound
expression can combine.
PiperOrigin-RevId: 269337222
cb3ecb5291