R/frame.R
    h2o.melt.RdPivot the frame designated by the three columns: index, column, and value. Index and column should be of type enum, int, or time. For cases of multiple indexes for a column label, the aggregation method is to pick the first occurrence in the data frame
h2o.melt( x, id_vars, value_vars = NULL, var_name = "variable", value_name = "value", skipna = FALSE )
| x | an H2OFrame | 
|---|---|
| id_vars | the columns used as identifiers | 
| value_vars | what columns will be converted to key-value pairs (optional, if not specified complement to id_vars will be used) | 
| var_name | name of the key-column (default: "variable") | 
| value_name | name of the value-column (default: "value") | 
| skipna | if enabled, do not include NAs in the result (default: FALSE) | 
an unpivoted H2OFrame