4. Fancy curves

By specifying || an arbitrary object may be used to construct the curved arrows. Indeed with a |.̇twocell| different objects can be used with the upper and lower curves by specifying || and ||.

These specifications apply to all 2-cells subsequently constructed at the same level of TEX grouping. Alternatively using a |~|-option, as in <#496#>Extra Options~2<#496#>, allows such a specification for a single 2-cell or curved part.

Objects used to construct curves can be of two types. Either a single ;SPMlt;object;SPMgt; is set once, with copies placed along the curve. Alternatively a directional object can be aligned with the tangent along the curve. In this case use a specification takes the form:

|<#497#>|;SPMlt;spacer;SPMgt;|~**|;SPMlt;object;SPMgt;|<#497#>|.

Here ;SPMlt;spacer;SPMgt; may be any ;SPMlt;object;SPMgt; of non-zero size. Typically it is empty space, |+|;SPMlt;dimen;SPMgt;|<#498#><#498#>|.

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#exercise499#