The interpretation of drawings, by breaking them into subshapes and classifying these subshapes, is an essential part of creative designing. Drawings must be open to different interpretations - i.e. different decompositions into parts, and classification of these parts in different ways - but conventional CAD systems do not readily allow this. Their data structures are too inflexible, and they do not provide subshape or implied-shape recognition capabilities. This paper discusses the centrality of emergent forms in the design process and proposes a datastructure based on construction lines and ordered lists which enables shapes as collection of lines and arcs to be efficiently encoded. The strategy to build a design tool around this data structure is also presented.