Identifying Cangjie components

Abun

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I have recently started to try and learn Cangjie but I still have quite a few problems discerning which parts should be omitted or not.

I know the basic rules of course: When it's two parts, take first and last sign of the first component as well as first, second and last of the second one; when it's three parts, take the first and last of the first two components and the last part of the third one. However in many cases I find it unclear where to draw the lines between components. Take 龍 for example: It could be analyzed as one unit (which would be historically correct), or two (立over月+rest) or three (立+月+rest). The first and third of theses alternatives would both render ytbp (卜廿月心), while analyzing it into two parts would give ybysp (卜月卜尸心). Now for some reason this last code is the correct one, although this way of analyzing actually makes the least sense to me because neither 立over月 nor the seven strokes on the right hand side of 龍 are legitimate characters.

Is there any kind of rule on how to identify the components in cases like this (and by extension, the correct parts to eliminate while typing)? Besides the "parts which intersect each other belong to the same component" rule of course, because that rule actually should have resulted in 龍 being analyzed into three parts instead of two.
 
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