2001
Although the two appraoches are not hard to understand , the operation and optimization of lattice structures is quite complicated , especially for large size structure design .
 concept or principle: optimization; engineering feature: lattice structures; engineering feature: structure design
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: optimization; engineering feature: lattice structures; engineering feature: structure design