src/main/java/org/fest/assertions/PropertySupport.java
@NotNull String firstPropertyIfNested(@NotNull String propertyName) {
    if (!isNestedProperty(propertyName)) {
      return propertyName;
    }
    return checkNotNull(propertyName.substring(0, propertyName.indexOf(SEPARATOR)));
  }
@NotNull String firstPropertyIfNested(@NotNull String propertyName) {
    if (!isNestedProperty(propertyName)) {
      return propertyName;
    }
    return checkNotNull(propertyName.substring(0, propertyName.indexOf(SEPARATOR)));
  }
Returns the first property from the given property name only if the given property name belongs to a nested
property. For example, given the nested property "address.street.name", this method will return "address". This
method returns the given property name unchanged if it does not belong to a nested property.

@param propertyName the given property name.
@return the first property from the given property name, if the property name belongs to a nested property;
otherwise, it will return the given property name unchanged.
@throws NullPointerException if given property name is {@code null}.