public static void validateMapInput(final Map map, final boolean ignoreTokens) {
        MergeElementStep.validate(map, ignoreTokens, allowedTokens, "mergeE");
    }
Implementation for the {@code mergeE()} step covering both the start step version and the one used mid-traversal.
gremlin-core/src/main/java/org/apache/tinkerpop/gremlin/process/traversal/step/map/MergeEdgeStep.java