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4 Category 2-Cells
 4.1 Attributes for the Type of 2-Cells
 4.2 Adding 2-Cells to a Category
 4.3 Identity 2-Cell and Composition of 2-Cells
 4.4 Well-Definedness for 2-Cells

4 Category 2-Cells

4.1 Attributes for the Type of 2-Cells

4.1-1 Source
‣ Source( c )( attribute )

Returns: a morphism

The argument is a \(2\)-cell \(c: \alpha \rightarrow \beta\). The output is its source \(\alpha\).

4.1-2 Range
‣ Range( c )( attribute )

Returns: a morphism

The argument is a \(2\)-cell \(c: \alpha \rightarrow \beta\). The output is its range \(\beta\).

4.1-3 Target
‣ Target( c )( attribute )

Returns: a morphism

The argument is a \(2\)-cell \(c: \alpha \rightarrow \beta\). The output is its target \(\beta\).

4.2 Adding 2-Cells to a Category

4.2-1 Add
‣ Add( category, twocell )( operation )

Adds twocell as a \(2\)-cell to category.

4.2-2 AddTwoCell
‣ AddTwoCell( category, twocell )( operation )

Adds twocell as a \(2\)-cell to category. If twocell already lies in the filter IsCapCategoryTwoCell, the operation Add (4.2-1) can be used instead.

4.2-3 CreateCapCategoryTwoCellWithAttributes
‣ CreateCapCategoryTwoCellWithAttributes( category, source, range[, attr1, val1, attr2, val2, ...] )( function )

Returns: a twocell

Creates a \(2\)-cell in category with the given attributes.

4.3 Identity 2-Cell and Composition of 2-Cells

4.3-1 IdentityTwoCell
‣ IdentityTwoCell( alpha )( attribute )

Returns: a \(2\)-cell

The argument is a morphism \(\alpha\). The output is its identity \(2\)-cell \(\mathrm{id}_{\alpha}: \alpha \rightarrow \alpha\).

4.3-2 HorizontalPreCompose
‣ HorizontalPreCompose( c, d )( operation )

Returns: a \(2\)-cell

The arguments are two \(2\)-cells \(c: \alpha \rightarrow \beta\), \(d: \gamma \rightarrow \delta\) between morphisms \(\alpha, \beta: a \rightarrow b\) and \(\gamma, \delta: b \rightarrow c\). The output is their horizontal composition \(d \ast c: (\gamma \circ \alpha) \rightarrow (\delta \circ \beta)\).

4.3-3 HorizontalPostCompose
‣ HorizontalPostCompose( d, c )( operation )

Returns: a \(2\)-cell

The arguments are two \(2\)-cells \(d: \gamma \rightarrow \delta\), \(c: \alpha \rightarrow \beta\) between morphisms \(\alpha, \beta: a \rightarrow b\) and \(\gamma, \delta: b \rightarrow c\). The output is their horizontal composition \(d \ast c: (\gamma \circ \alpha) \rightarrow (\delta \circ \beta)\).

4.3-4 VerticalPreCompose
‣ VerticalPreCompose( c, d )( operation )

Returns: a \(2\)-cell

The arguments are two \(2\)-cells \(c: \alpha \rightarrow \beta\), \(d: \beta \rightarrow \gamma\) between morphisms \(\alpha, \beta, \gamma: a \rightarrow b\). The output is their vertical composition \(d \circ c: \alpha \rightarrow \gamma\).

4.3-5 VerticalPostCompose
‣ VerticalPostCompose( d, c )( operation )

Returns: a \(2\)-cell

The arguments are two \(2\)-cells \(d: \beta \rightarrow \gamma\), \(c: \alpha \rightarrow \beta\) between morphisms \(\alpha, \beta, \gamma: a \rightarrow b\). The output is their vertical composition \(d \circ c: \alpha \rightarrow \gamma\).

4.4 Well-Definedness for 2-Cells

4.4-1 IsWellDefinedForTwoCells
‣ IsWellDefinedForTwoCells( c )( operation )

Returns: a boolean

The argument is a \(2\)-cell \(c\). The output is true if \(c\) is well-defined, otherwise the output is false.

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