7.7.8 Operations on tagged maps

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The most important operations on maps are:

x mapp (x mapp)
True when x is a tagged map. False if x is tagged data which is not a tagged map. Examples: map ( bottom ) mapp is true and 117 mapp is false.
x catching maptag (x catching maptag)
Same as map ( x ) but also normalizes x before putting a tag on it.
x maptag (x maptag)
Same as x catching maptag but if x turns out to be an exception then x maptag returns the exception as it is.
x untag (x untag)
If x is a tagged map then x untag removes the tag and normalizes the result.
x apply y (x apply y)
If x and y are tagged maps then x apply y removes the tag from both of them, applies the former to the latter, and puts a MapTag on the result. In other words, x apply y performs the x ' y operation 'under the MapTag'. Note that x apply y in itself does very little work since it applies x to y in a lazy way. The actual work connected with applying x to y is done when forced.

x root (x root)
If x is a tagged map then x root removes the MapTag and reduces the raw map to root normal form. If this results in a true normal form then x root returns true. If reduction results in a function normal form then x root returns false.

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