Clearance 余地、余隙、空环 指多层板之各内层上，若不欲其导体面与通孔之孔壁连通时，则可将通孔周围的铜箔蚀掉而形成空环，特称为“空环”。又外层板面上所印的绿漆与各孔环之间的距离也称为 Clearance 。不过由于目前板面线路密度愈渐提高，使得这种绿漆原有的余地也被紧逼到几近于无了。
本文对“空环”的阐释理据主要包括两个方面：认知经济原则和信息度理论。
In this thesis, the theoretical base of missing links is discussed in terms of two theories: cognitive economy principle and informativity theory.
驾驶舱门(防劫机系统)被大大加强，更多的空军力量保持着在空巡逻，出入境这一环节上也鲜有差池。
Cockpit doors have been reinforced, more air marshals patrol the skies, and better records are kept on those entering the country.
我一定要在醉花丛中氤氲的树影里寻找，在那里，鸽子在它们的角落里咕咕地叫着，精灵的脚环在繁星满空的静夜里叮当作响。
I must search in the drowsy shade of the bakula grove, where pigeons coo in their corner, and fairies' anklets tinkle in the stillness of starry nights.
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