Applies transformation rules to the parts of an expression at the given
ReplaceAt[expr, rules, n]
ReplaceAt[expr, rules, {i, j, …}]
ReplaceAt[expr, rules, {{i1, j1, …}, {i2, j2, …}, …}]
ReplaceAt[rules, pos]
ReplaceAt[{a, a, a, a}, a -> xx, 2]
→ {a, xx, a, a}ReplaceAt[{a, a, a, a}, a -> xx, {{1}, {4}}]
→ {xx, a, a, xx}ReplaceAt[{{a, a}, {a, a}}, a -> xx, {2, 1}]
→ {{a, a}, {xx, a}}ReplaceAt[{1, 2, 3, 4}, x_ :> 2 x - 1, {{2}, {4}}]
→ {1, 3, 3, 7}ReplaceAt[{a, b, c, d}, {a -> xx, _ -> yy}, {{1}, {2}, {4}}]
→ {xx, yy, c, yy}ReplaceAt[a -> xx, 2][{a, a, a, a}]
→ {a, xx, a, a}