validate_element is a local copy of the ggplot2 function which checks the validity of a given theme element
against the elements table. Since the .elements_tree is an internal function, which is not exported, and modifications could not be made,
a new (and equivalent) .element_tree is created within ggtern to handle the new theme elements created within this package.
plot_theme is a local copy of the method that determines the net theme between a plot and the current global theme.
add_theme is a local copy of the ggplot2 function which modifies the current theme, by a proposed theme.
It is slightly modified to handle 'logical' values the same way it handles 'character' or 'numeric' values,
which do not inherit from 'element' objects.
"%+replace%" is a local copy of the ggplot2 replace operator, no different other than being exported from the ggtern namespace.
update_theme is a local copy of a ggplot2 function, which copies elements from the new theme into an old theme.
combine_elements is a local copy of the ggplot2 function that combines two theme elements
calc_element is a local copy of the ggplot2 function which determines the net element based on inheritances, given input theme.
validate_element(el, elname)
theme_update(...)
plot_theme(x)
add_theme(t1, t2, t2name)
e1 %+replace% e2
update_theme(oldtheme, newtheme)
combine_elements(e1, e2)
calc_element(element, theme, verbose = FALSE)t1theme_update