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.