Structural manipulation is not just about manipulating mathematical formulae algebraically but is also about adding/removing/replacing parts of expressions structurally, often without evaluating (parts of) the expression. Structural manipulation usually involves functions of the
Hold family (
HoldPattern), controlled evaluation (
ReleaseHold) and replacement functions (
For strict algebraic manipulation, use algebraic-manipulation or simplifying-expressions. If your question is about how to withhold subexpressions from evaluation or how to evaluate only parts of expressions, use (or add) evaluation. Questions explicitly about replacing subexpressions should use replacement.