A left-transform is a side-transform that extends a binary-operation AST toward the left by replacing an AST object with a larger binary operation. section 12.5
Side-transforms support character-by-character expression editing, reducing domain-expert actions and avoiding the need to plan an entire expression before creating it. section 12.5
Building an expression incrementally
The expression 1 + 2 illustrates incremental AST construction: an existing operand becomes part of a binary operation, leaving the other operand to be filled. section 12.4 section 12.5
