There is a special form for boolean formulas called "Conjunctive Normal Form" (CNF). A problem in this form consists of clauses connected with and operators, where each clause only contains variables connected with or operators. The variables can appear negated, but only variables can be directly negated, something like !(a && b) is not allowed. An example boolean formula in CNF form is:
（四）出于他人胁迫或者诱骗的；
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Aurora lights shimmering over Earth were filmed on camera by Russia's space agency Roscosmos cosmonaut Sergey Kud-Sverchkov from the International Space Station. The phenomenon is caused by solar storms emitting high-speed charged particles colliding with gases in Earth’s atmosphere. The most common colour seen during this display is green, although other colours such as pink and red are sometimes visible as well.