Note that there must be the same number of symbols on both the left and right sides of the rule. Rules that are applied to symbols must consider the "context" of the symbol.
For example, in the previous rule, a "B" can be turned into an "X" if (but not necessarily only if) it has a preceding "A" and a following "C". This makes it difficult to modify assumptions since individual symbols cannot be interpreted on their own. Also, the number of rules needed to fully specify the grammar might become very large, since there could be many special cases that need to be considered.
We will continue this topic in the next post..
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