replace 'nil' by 'leer' since 'nil' leads to conflict with macOS.

src/classes/PFunction.cpp

@@ -130,7 +130,7 @@ Bool_t PFunction::FindAndCheckMapAndParamRange(const musrfit::ast::operand& oper
     CheckVisitor(UInt_t mapSize, UInt_t paramSize, PFunction* func)
       : fMapSize(mapSize), fParamSize(paramSize), fFunc(func) {}
 
-    Bool_t operator()(const musrfit::ast::nil&) const { return true; }
+    Bool_t operator()(const musrfit::ast::leer&) const { return true; }
     Bool_t operator()(double) const { return true; }
     Bool_t operator()(const musrfit::ast::constant&) const { return true; }
 
@@ -322,7 +322,7 @@ TString PFunction::GenerateStringOperand(const musrfit::ast::operand& operand)
     PFunction* fFunc;
     StringVisitor(PFunction* func) : fFunc(func) {}
 
-    TString operator()(const musrfit::ast::nil&) const { return "nil"; }
+    TString operator()(const musrfit::ast::leer&) const { return "leer"; }
 
     TString operator()(double val) const {
       std::ostringstream oss;
@@ -419,11 +419,11 @@ TString PFunction::GenerateStringOperand(const musrfit::ast::operand& operand)
 //==========================================================================
 
 /**
- * \brief Evaluates a nil (empty) AST node.
+ * \brief Evaluates a leer (empty) AST node.
  *
  * \return Always returns 0.0
  */
-Double_t PFunction::EvalVisitor::operator()(const musrfit::ast::nil&) const
+Double_t PFunction::EvalVisitor::operator()(const musrfit::ast::leer&) const
 {
   return 0.0;
 }

src/include/PFunction.h

@@ -184,7 +184,7 @@ class PFunction {
                   const PMetaData& metaData)
         : fMap(map), fParam(param), fMetaData(metaData) {}
 
-      Double_t operator()(const musrfit::ast::nil&) const;
+      Double_t operator()(const musrfit::ast::leer&) const;
       Double_t operator()(double val) const;
       Double_t operator()(const musrfit::ast::constant& c) const;
       Double_t operator()(const musrfit::ast::parameter& p) const;

src/include/PFunctionAst.h

@@ -107,7 +107,7 @@ namespace musrfit { namespace ast
      *
      * Used as a placeholder in variant types where no value is present.
      */
-    struct nil {};
+    struct leer {};
 
     // Forward declarations for recursive AST structures
     struct expression;
@@ -173,7 +173,7 @@ namespace musrfit { namespace ast
      * recursive grammar structures.
      */
     typedef boost::variant<
-            nil                                      ///< Empty placeholder
+            leer                                     ///< Empty placeholder
           , double                                   ///< Numeric literal
           , constant                                 ///< Constant value
           , parameter                                ///< Parameter reference
@@ -246,11 +246,11 @@ namespace musrfit { namespace ast
     };
 
     /**
-     * @brief Stream output operator for nil nodes (debugging support).
+     * @brief Stream output operator for leer nodes (debugging support).
      * @param out the output stream
      * @return the output stream for chaining
      */
-    inline std::ostream& operator<<(std::ostream& out, nil) { out << std::string("nil"); return out; }
+    inline std::ostream& operator<<(std::ostream& out, leer) { out << std::string("leer"); return out; }
 }}
 
 // Boost.Fusion adaptations to make structures compatible with X3 attribute propagation