diff --git a/Makefile b/Makefile
index 861dbd4..c8e930a 100644
--- a/Makefile
+++ b/Makefile
@@ -28,61 +28,61 @@ solutions: $(TSTP_BASIC) \
 					 $(TSTP_BICOND) \
 					 $(TSTP_NEG)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: basic
 basic: $(TSTP_BASIC)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: conjunction
 conjunction: $(TSTP_CONJ)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: disjunction
 disjunction: $(TSTP_DISJ)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: implication
 implication: $(TSTP_IMPL)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: biconditional
 biconditional: $(TSTP_BICOND)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 .PHONY: negation
 negation: $(TSTP_NEG)
 	@find . -type f -name "cnf*" -delete
-	@echo "ATP: $(ATP)"
+	@echo "ATP: ${ATP}"
 
 problems/basic/%.tstp: problems/basic/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 problems/conjunction/%.tstp: problems/conjunction/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 problems/disjunction/%.tstp: problems/disjunction/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 problems/implication/%.tstp: problems/implication/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 problems/biconditional/%.tstp: problems/biconditional/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 problems/negation/%.tstp: problems/negation/%.tptp
 	@echo $@
-	@$(ATP) $< > $@
+	@${ATP} $< > $@
 
 clean:
 	find . -type f -name "*.aux" -delete