Given types $A$ and $B$, we can combine them to get the product types $A\times B$ and the sum type $A \oplus B$. These are both inductive types parametrized by $A$ and $B$.
The Product
A term in the product $A \times B$ corresponds to an element of $A$ and an element of $B$. Hence we can define the product using a single constructor. We use square brackets for pairs as parenthesis have other meanings in Agda.
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The (direct) Sum
The direct sum $A \oplus B$ of types $A$ and $B$ can be thought of as the type whose elements are the elements of $A$ and the elements of $B$. Formally we have inclusion maps $\iota_j$ from the given types. Hence we get a description with two constructors.
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