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Here a primary ground structure optimisation is followed by a rectification process in which elements falling below a certain minimum crosB-sections threshold are eliminated , before a final optimisation step is conducted , for which the method of Feasible Directions is employed .
 concept or principle: structure; concept or principle: process; material: elements; concept or principle: crosB-sections; concept or principle: step
Please list all entity words in the text that fit the category.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
Answer:
 concept or principle: structure; concept or principle: process; material: elements; concept or principle: crosB-sections; concept or principle: step