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  17 <H3><A NAME="SEC18" HREF="suif1_toc.html#TOC18">Block Nodes</A></H3>
  18 <P>
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  26 A <CODE>tree_block</CODE> node introduces a new scope.  Nested scopes are
  27 useful for retaining scope information from source programs and for
  28 debugging purposes.  They are also very useful for implementing code
  29 transformations.  For example, name conflicts are easily avoided by
  30 introducing new scopes.
  31
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  33 <P>
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  38 A <CODE>tree_block</CODE> node contains a <CODE>block_symtab</CODE> symbol table and
  39 a tree node list for the body.  The symbol table is accessed with the
  40 <CODE>symtab</CODE> and <CODE>set_symtab</CODE> methods.  The <CODE>body</CODE> and
  41 <CODE>set_body</CODE> methods are used to get and set the list of tree nodes
  42 inside the block.
  43
  44 </P>
  45 <P>
  46 There is a one-to-one correspondence between a <CODE>tree_block</CODE> and its
  47 symbol table, and the hierarchy of symbol tables within a procedure must
  48 parallel the hierarchy of <CODE>tree_block</CODE> nodes.  When inserting a
  49 <CODE>tree_block</CODE> into an AST, the new block's symbol table must be
  50 inserted in the appropriate place in the symbol table hierarchy.  When a
  51 <CODE>tree_block</CODE> is destroyed, the associated symbol table is detached
  52 from its parent table and destroyed.  However, the converse is not
  53 true--when a <CODE>block_symtab</CODE> is deleted, the corresponding
  54 <CODE>tree_block</CODE> node is not deleted.
  55
  56 </P>
  57 <P>
  58 The entries in a block's symbol table may not be referenced from outside
  59 the block.  There are no other restrictions on <CODE>tree_block</CODE> nodes.
  60 The bodies may be empty or contain any kinds of tree nodes.  Blocks are
  61 usually entered from the beginning but that is not required; unlike
  62 other AST nodes, branches into the middle of a block are allowed.
  63
  64 </P>
  65 <P>
  66 As an example, the following code creates a new <CODE>tree_block</CODE> and
  67 its associated symbol table.  It then adds the new block to the
  68 body of an existing block node:
  69
  70 </P>
  71
  72 <PRE>
  73 block_symtab *new_symtab = new block_symtab("my_symtab");
  74 cur_block-&#62;symtab()-&#62;add_child(new_symtab);
  75 tree_node_list *tl = new tree_node_list;
  76 tree_block *new_block = new tree_block(tl, new_symtab);
  77 cur_block-&#62;body()-&#62;append(new_block);
  78 </PRE>
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