parsing - 最左推导是什么意思？

Question

请帮我理解LL Parser中第二个L的最左推导的含义。

用一个最简单的例子来解释一下。

我看到下图解释了最左推导，但我不明白:

Answer 1

语法规则显示在左侧，包括非终结符和终结符。非终结符应该是大写字母，其他所有符号通常都是终结符。在示例中，N 和 D 是非终结符，0-9 是终结符。最左推导总是使最左非终结符经过语法规则。尝试格式化下面的示例。

N
=> N D   --Replaces the first/left most/only (which is "N") with the N => N D rule
=> N D D --Replaces the first/left most nonterminal (which is "N") with the N => N D rule
=> D D D --Replaces the first nonterminal (which is "N") with the N => D rule
=> 1 D D --Replaces the first nonterminal ("D") with the D => 1 rule(our first terminal character!)
=> 1 2 D --Replaces the first nonterminal ("D") with the D => 2 rule
=> 1 2 3 --Replaces the first nonterminal ("D") with the D => 3 rule
-- Only terminal characters remain, derivation/reduction is complete.