LEAF ENUM TYPE issue: -ve value not accepted. as per RFC, its should accept

yang file:
leaf ifType {
type enumeration {
enum "unbounded";
enum ZERO;
enum two;
enum four;
enum seven

}
}

RFC6020:
9.6.4.2. The value Statement

The "value" statement, which is optional, is used to associate an
integer value with the assigned name for the enum. This integer
value MUST be in the range -2147483648 to 2147483647, and it MUST be
unique within the enumeration type. The value is unused by YANG and
the XML encoding, but is carried as a convenience to implementors.

If a value is not specified, then one will be automatically assigned.
If the "enum" substatement is the first one defined, the assigned
value is zero (0); otherwise, the assigned value is one greater than
the current highest enum value.

If the current highest value is equal to 2147483647, then an enum
value MUST be specified for "enum" substatements following the one
with the current highest value.

[INFO] — yangutils-maven-plugin:1.0.0-SNAPSHOT:yang2java (default) @ onlab-rest —
[INFO] Error in file: /home/root1/onos/utils/test/src/main/yang/Onos_Yang_1.yang at line: 49 at position: 23
no viable alternative at input 'value -2147483647'
[INFO]
org.onosproject.yangutils.parser.exceptions.ParserException: no viable alternative at input 'value -2147483647'
at org.onosproject.yangutils.parser.impl.parserutils.ParseTreeErrorListener.syntaxError(ParseTreeErrorListener.java:36)
at org.antlr.v4.runtime.ProxyErrorListener.syntaxError(ProxyErrorListener.java:65)
at org.antlr.v4.runtime.Parser.notifyErrorListeners(Parser.java:558)
at org.antlr.v4.runtime.DefaultErrorStrategy.reportNoViableAlternative(DefaultErrorStrategy.java:310)
at org.antlr.v4.runtime.DefaultErrorStrategy.reportError(DefaultErrorStrategy.java:147)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.enumStatementBody(GeneratedYangParser.java:9579)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.enumStatement(GeneratedYangParser.java:8533)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.enumSpecification(GeneratedYangParser.java:8465)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.typeBodyStatements(GeneratedYangParser.java:6950)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.typeStatement(GeneratedYangParser.java:6866)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.leafStatement(GeneratedYangParser.java:12137)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.dataDefStatement(GeneratedYangParser.java:6546)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.bodyStatements(GeneratedYangParser.java:1818)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.moduleBody(GeneratedYangParser.java:365)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.moduleStatement(GeneratedYangParser.java:304)
at org.onosproject.yangutils.parser.antlrgencode.GeneratedYangParser.yangfile(GeneratedYangParser.java:242)
at org.onosproject.yangutils.parser.impl.YangUtilsParserManager.getDataModel(YangUtilsParserManager.java:75)
at org.onosproject.yangutils.plugin.manager.YangUtilManager.execute(YangUtilManager.java:148)
at org.apache.maven.plugin.DefaultBuildPluginManager.executeMojo(DefaultBuildPluginManager.java:134)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:208)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:153)
at org.apache.maven.lifecycle.internal.MojoExecutor.execute(MojoExecutor.java:145)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:116)
at org.apache.maven.lifecycle.internal.LifecycleModuleBuilder.buildProject(LifecycleModuleBuilder.java:80)
at org.apache.maven.lifecycle.internal.builder.singlethreaded.SingleThreadedBuilder.build(SingleThreadedBuilder.java:51)
at org.apache.maven.lifecycle.internal.LifecycleStarter.execute(LifecycleStarter.java:128)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:307)
at org.apache.maven.DefaultMaven.doExecute(DefaultMaven.java:193)
at org.apache.maven.DefaultMaven.execute(DefaultMaven.java:106)
at org.apache.maven.cli.MavenCli.execute(MavenCli.java:862)
at org.apache.maven.cli.MavenCli.doMain(MavenCli.java:286)
at org.apache.maven.cli.MavenCli.main(MavenCli.java:197)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.codehaus.plexus.classworlds.launcher.Launcher.launchEnhanced(Launcher.java:289)
at org.codehaus.plexus.classworlds.launcher.Launcher.launch(Launcher.java:229)
at org.codehaus.plexus.classworlds.launcher.Launcher.mainWithExitCode(Launcher.java:415)
at org.codehaus.plexus.classworlds.launcher.Launcher.main(Launcher.java:356)
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------