继续翻译

The previous example shows two levels of nesting, but any number oflevels is possible.  For example, here are three levels:     x = y     y = z     z = u     a := $($($(x)))Here the innermost `$(x)' expands to `y', so `$($(x))' expands to`$(y)' which in turn expands to `z'; now we have `$(z)', which becomes`u'.   References to recursively-expanded variables within a variable nameare reexpanded in the usual fashion.  For example:     x = $(y)     y = z     z = Hello     a := $($(x))defines `a' as `Hello': `$($(x))' becomes `$($(y))' which becomes`$(z)' which becomes `Hello'.   Nested variable references can also contain modified references andfunction invocations (*note Functions for Transforming Text:Functions.), just like any other reference.  For example, using the`subst' function (*note Functions for String Substitution and Analysis:Text Functions.):     x = variable1     variable2 := Hello     y = $(subst 1,2,$(x))     z = y     a := $($($(z)))eventually defines `a' as `Hello'.  It is doubtful that anyone wouldever want to write a nested reference as convoluted as this one, but itworks: `$($($(z)))' expands to `$($(y))' which becomes `$($(subst1,2,$(x)))'.  This gets the value `variable1' from `x' and changes itby substitution to `variable2', so that the entire string becomes`$(variable2)', a simple variable reference whose value is `Hello'.

前面的例子展示了两层的嵌套，但是任何层的嵌套都是可能的。例如，这里有三个层次：

x = y

y = z

z = u

a := $($($(x)))

这里，最里层的 `$(x)' 扩展到  `y'，于是 `$($(x))' 扩展到`$(y)'，进而扩展到`z'; 现在我们拥有`$(z)'，它将变为 `u'。

在一个变量里面，对递归式扩展的变量的参照会合正常的扩展一样。例如:

x = $(y)

y = z

z = Hello

a := $($(x))

定义 `a' 为 `Hello': `$($(x))' 为 `$($(y))' ，然后转化为，`$(z)' ，再转化为 `Hello'。

嵌套的变量参照也能包含变化的参照和函数激活(*note Functiosn for Transforming Text: Functions)，就像其他的参照一样。例如，使用 subst函数(*Functions for String Substitution and Analysis: Text Functions.):

x = variable1

variable2 := Hello

y = $(subst 1,2,$(x))

z = y

a := $($($(z)))

最终定义 `a' 为 `Hello'。是否会有人写出这样的嵌套式的参照是令人怀疑的，但是它确实可以工作：

`$($($(z)))' 扩展为 `$($(y))' ，它变成 `$($(subst

1,2,$(x)))'. 然后从x 来获得 `variable1' ,再通过变换，把它变成 `variable2', 这样整个字符串就变成了

`$(variable2)', 成为一个简单的变量参照，其值为 `Hello'。

后文待续