UML的词汇表包含3种构造块，但不包括下面的(52)；UML中有　4种事物，但不包括下面的(53)；UML中有4种关系，但不包括下面的(54)。

(36)

(37)

(34)

(35)

Basically,　a(71)is　a　standalone　process　or　a　set　of　integrated　processes　that　runs　on　a　router　or　server　to　control　the　flow　of(72)application　traffic(73)through　it.　Typically,　firewalls　are(74)on　the　entry　point　to　a　public　network　such　as　the　Internet.　They　could　be　considered　traffic　cops.　The　firewall'　s(75)is　to　ensure　that　all　communication　between　an　organization's　network　and　the　Internet　conform.　to　the　organization's　security　policies.

(32)

(31)

(30)

(29)

Software　design　is　a(66)process.　It　requires　a　certain(67)of　flair　on　the　part　of　the　designer.　Design　can　net　be　learned　from　a　book.　It　must　be　practiced　and　learnt　by　experience　and　study　of　existing　systems.　A　well(68)software　system　is　straight　forward　to　implement　and　maintain,　easily(69)and　reliable.　Badly(68)software　systems,　although　they　may　work　are(70)to　be　expensive　to　maintain,　difficult　to　test　and　unreliable.