Map f at the data of the second child of a tree:

Map at any position:

Map at several positions:

Use the operator form of TreeMapAt:

TreeMap[f,tree,levelspec] is equivalent to TreeMapAt[f,tree,TreeLevel[levelspec]]:

Construct a tree from the heads in an expression:

TreeMapAt maps at the data of subtrees of a tree:

This corresponds to mapping at the heads and leaves in an expression:

MapAt can map at the heads and leaves directly:

If pos is not a list, pos and {pos} are equivalent specifications:

TreeMapAt[f,tree,{{i₁,j₁,…},{i₂,j₂,…},…}] is equivalent to …@TreeMapAt[f,{i₂,j₂,…}]@TreeMapAt[f,{i₁,j₁,…}]@tree:

This extends to the empty list:

Mapping at position {} applies the function to the data of the root node:

TreeMapAt applies f repeatedly if a position is mentioned repeatedly:

TreeMapAt can use the lists of positions returned by TreePosition:

This is the data returned by TreeExtract:

TreeMapAt[f,tree,{p₁,p₂,…}] treats {p₁,p₂,…} as a list of individual position specifications if all the p_i are lists:

For {{1,2},{3,4}}, the data at positions {1,2} and {3,4} is modified:

If any p_i is not a list, {p₁,p₂,…} is treated as a list of part specifications:

For {{{1,2},{3,4}}}, parts 3 and 4 of parts 1 and 2 are modified:

TreeMapAt[f,tree,{}] does not map f because the list of positions is empty:

TreeMapAt[f,tree,{{}}] maps f at position {}, which corresponds to the root node:

A list of lists in the second argument is always interpreted as a list of position specifications:

As a result, specifications that are equivalent in Part may be treated differently by TreeMapAt:

Use a list of lists if consistency between Part and TreeMapAt is needed: