在数据库系统中，若某基本层次联系的集合满足以下两个条件：允许一个以上的结点无双亲；一个结点可以有多于一个双亲，则该集合称为(43)。

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Currently　the　Pentium　chip　or(71),　made　by(72),　is　the　most　common　CPU　though　there　are　many　other　companies　that　produce　processors　for　personal　computers.　One　example　is　the　CPU　made　by　Motorola　which　is　used　in　Apple　computers.　With　faster　processors　the　clock　speed　becomes　more　important.　Compared　to　some　of　the　first　computers　which　operated　at　below　30　megahertz((73))the　Pentium　chips　began　at　75　megahertz　in　the　late　1990g.　As　of　mid　2003　speeds　now　exceed　3000　+　megahertz　or　3　gigahertz(check　your　local　computer　store　for　the　latest　speed).　It　depends　on　the(74)that　the　chip　is　housed　in,　or　the(75),　as　to　whether　you　are　able　to　upgrade　to　a　faster　chip.　The　motherboard　contains　the　circuitry　and　connections　that　allow　the　various　component　to　communicate　with　each　other.

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Another　aspect　of　C　that's　worth　mentioning　here　is　that　it　is,　to　put　it　bluntly,　a　bit　dangerous.　C　does　not,　in　general,　try　hard　to　protect　a　programmer　from　mistakes.　If　you　write　a　piece　of(66)which　will(through　some　oversight　of　yours)do　something　wildly　different　from　what　you　intended　it　to　do,　up　to　and　including(67)your(68)or　trashing　your(69),　and　if　it　is　possible　for　the(70)to　compile　it,　it　generally　will.　You　wont　get　warnings　of　the　form.　"Do　you　really　mean　to...　?"　or　"Are　you　sure　you　really　want　to...　?".　C　is　often　compared　to　a　sharp　knife　:'　it　can　do　a　surgically　precise　job　on　some　exacting　task　you　have　in　mind,　but　it　can　also　do　a　surgically　precise　job　of　cutting　off　your　finger.　It's　up　to　you　to　use　it　carefully.