Equation type is the type of variable that equation can solve. Equation type is calculated in symbolic by both variability and basic type of its sub-expressions. Generally; type calculation is a bottom-up way as expression is represented in form of tree. But; there are kinds of particular expressions; such as integer(); noEvent(); multi-output function call expression; etc; which may cause type and variability incompatible problem. The issue is discussed in the paper; and several rules for variability and type calculation are present. These rules will helps to debug out obscure errors; and several typical examples are present to show how the rules work.
Keywords: Equation type; equation variability; compatibility of variability and type; model debug
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