▁use ▁a ▁standard ▁rule ▁of ▁det achment ." ▁ ▁A ▁substitution ▁A ▁that ▁when ▁applied ▁to ▁ ▁produces ▁, ▁and ▁substitution ▁B ▁that ▁when ▁applied ▁to ▁ ▁produces ▁, ▁are ▁called ▁un ifiers ▁of ▁ ▁and ▁. ▁ ▁Vari ous ▁un ifiers ▁may ▁produce ▁expressions ▁with ▁varying ▁numbers ▁of ▁free ▁variables . ▁ ▁Some ▁possible ▁un ifying ▁expressions ▁are ▁substitution ▁instances ▁of ▁others . ▁ ▁If ▁one ▁expression ▁is ▁a ▁substitution ▁instance ▁of ▁another ▁( and ▁not ▁just ▁a ▁variable ▁ren aming ), ▁then ▁that ▁other ▁is ▁called ▁" more ▁general ". ▁ ▁If ▁the ▁most ▁general ▁un ifier ▁is ▁used ▁in ▁cond ensed ▁det achment , ▁then ▁the ▁logical ▁result ▁is ▁the ▁most ▁general ▁conclusion ▁that ▁can ▁be ▁made ▁in ▁the ▁given ▁inference ▁with ▁the ▁given ▁second ▁expression . ▁Since ▁any ▁we aker ▁inference ▁you ▁can ▁get ▁is ▁a ▁substitution ▁instance ▁of ▁the ▁most ▁general ▁one , ▁nothing ▁less ▁than ▁the ▁most ▁general ▁un ifier ▁is ▁ever ▁used ▁in ▁practice . ▁ ▁Some ▁log ics , ▁such ▁as ▁classical ▁propos itional ▁calculus , ▁have ▁a ▁set ▁of ▁defining ▁ax i oms ▁with ▁the ▁" D - comple ten ess " ▁property . ▁If ▁a ▁set ▁of ▁ax i oms ▁is ▁D - Complete , ▁then ▁any ▁valid ▁theorem ▁of ▁the ▁system , ▁including ▁all ▁of ▁its ▁substitution ▁instances ▁( up ▁to ▁variable ▁ren aming ), ▁can ▁be ▁generated ▁by ▁cond ensed ▁det achment ▁alone . ▁ ▁For ▁example , ▁if ▁ ▁is ▁a ▁theorem ▁of ▁a ▁D - complete ▁system , ▁cond