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

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